Michael S. Tsirkin
2021-Dec-15 21:33 UTC
vdpa legacy guest support (was Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero)
On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote:> > > On 12/14/2021 6:06 PM, Jason Wang wrote: > > On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <si-wei.liu at oracle.com> wrote: > > > > > > > > > On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote: > > > > On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote: > > > > > On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote: > > > > > > On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote: > > > > > > > Sorry for reviving this ancient thread. I was kinda lost for the conclusion > > > > > > > it ended up with. I have the following questions, > > > > > > > > > > > > > > 1. legacy guest support: from the past conversations it doesn't seem the > > > > > > > support will be completely dropped from the table, is my understanding > > > > > > > correct? Actually we're interested in supporting virtio v0.95 guest for x86, > > > > > > > which is backed by the spec at > > > > > > > https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure > > > > > > > if there's request/need to support wilder legacy virtio versions earlier > > > > > > > beyond. > > > > > > I personally feel it's less work to add in kernel than try to > > > > > > work around it in userspace. Jason feels differently. > > > > > > Maybe post the patches and this will prove to Jason it's not > > > > > > too terrible? > > > > > I suppose if the vdpa vendor does support 0.95 in the datapath and ring > > > > > layout level and is limited to x86 only, there should be easy way out. > > > > Note a subtle difference: what matters is that guest, not host is x86. > > > > Matters for emulators which might reorder memory accesses. > > > > I guess this enforcement belongs in QEMU then? > > > Right, I mean to get started, the initial guest driver support and the > > > corresponding QEMU support for transitional vdpa backend can be limited > > > to x86 guest/host only. Since the config space is emulated in QEMU, I > > > suppose it's not hard to enforce in QEMU. > > It's more than just config space, most devices have headers before the buffer. > The ordering in datapath (data VQs) would have to rely on vendor's support. > Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays > can/should well support the case when ORDER_PLATFORM is not acked by the > driver (actually this feature is filtered out by the QEMU vhost-vdpa driver > today), even with v1.0 spec conforming and modern only vDPA device. The > control VQ is implemented in software in the kernel, which can be easily > accommodated/fixed when needed. > > > > > > QEMU can drive GET_LEGACY, > > > GET_ENDIAN et al ioctls in advance to get the capability from the > > > individual vendor driver. For that, we need another negotiation protocol > > > similar to vhost_user's protocol_features between the vdpa kernel and > > > QEMU, way before the guest driver is ever probed and its feature > > > negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call > > > from the device, but we can assume weak ordering for legacy at this > > > point (x86 only)? > > I'm lost here, we have get_features() so: > I assume here you refer to get_device_features() that Eli just changed the > name. > > > > 1) VERSION_1 means the device uses LE if provided, otherwise natvie > > 2) ORDER_PLATFORM means device requires platform ordering > > > > Any reason for having a new API for this? > Are you going to enforce all vDPA hardware vendors to support the > transitional model for legacy guest? meaning guest not acknowledging > VERSION_1 would use the legacy interfaces captured in the spec section 7.4 > (regarding ring layout, native endianness, message framing, vq alignment of > 4096, 32bit feature, no features_ok bit in status, IO port interface i.e. > all the things) instead? Noted we don't yet have a set_device_features() > that allows the vdpa device to tell whether it is operating in transitional > or modern-only mode. For software virtio, all support for the legacy part in > a transitional model has been built up there already, however, it's not easy > for vDPA vendors to implement all the requirements for an all-or-nothing > legacy guest support (big endian guest for example). To these vendors, the > legacy support within a transitional model is more of feature to them and > it's best to leave some flexibility for them to implement partial support > for legacy. That in turn calls out the need for a vhost-user protocol > feature like negotiation API that can prohibit those unsupported guest > setups to as early as backend_init before launching the VM.Right. Of note is the fact that it's a spec bug which I hope yet to fix, though due to existing guest code the fix won't be complete. WRT ioctls, One thing we can do though is abuse set_features where it's called by QEMU early on with just the VERSION_1 bit set, to distinguish between legacy and modern interface. This before config space accesses and FEATURES_OK. Halil has been working on this, pls take a look and maybe help him out.> > > > > > > > I > > > > > checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level > > > > > 0.95 support, it seems all the ingredient had been there already dated back > > > > > to the DPDK days. The only major thing limiting is in the vDPA software that > > > > > the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for > > > > > a few DMA setup ops, which is virtio 1.0 only. > > > > > > > > > > > > 2. suppose some form of legacy guest support needs to be there, how do we > > > > > > > deal with the bogus assumption below in vdpa_get_config() in the short term? > > > > > > > It looks one of the intuitive fix is to move the vdpa_set_features call out > > > > > > > of vdpa_get_config() to vdpa_set_config(). > > > > > > > > > > > > > > /* > > > > > > > * Config accesses aren't supposed to trigger before features are > > > > > > > set. > > > > > > > * If it does happen we assume a legacy guest. > > > > > > > */ > > > > > > > if (!vdev->features_valid) > > > > > > > vdpa_set_features(vdev, 0); > > > > > > > ops->get_config(vdev, offset, buf, len); > > > > > > > > > > > > > > I can post a patch to fix 2) if there's consensus already reached. > > > > > > > > > > > > > > Thanks, > > > > > > > -Siwei > > > > > > I'm not sure how important it is to change that. > > > > > > In any case it only affects transitional devices, right? > > > > > > Legacy only should not care ... > > > > > Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the > > > > > modern one in a transitional device model rather than being legacy only. > > > > > That way a v0.95 and v1.0 supporting vdpa parent can support both types of > > > > > guests without having to reconfigure. Or are you suggesting limit to legacy > > > > > only at the time of vdpa creation would simplify the implementation a lot? > > > > > > > > > > Thanks, > > > > > -Siwei > > > > I don't know for sure. Take a look at the work Halil was doing > > > > to try and support transitional devices with BE guests. > > > Hmmm, we can have those endianness ioctls defined but the initial QEMU > > > implementation can be started to support x86 guest/host with little > > > endian and weak memory ordering first. The real trick is to detect > > > legacy guest - I am not sure if it's feasible to shift all the legacy > > > detection work to QEMU, or the kernel has to be part of the detection > > > (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking > > > effort in QEMU) as well. Let me take a further look and get back. > > Michael may think differently but I think doing this in Qemu is much easier. > I think the key is whether we position emulating legacy interfaces in QEMU > doing translation on top of a v1.0 modern-only device in the kernel, or we > allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a > transitional model in the kernel that is able to work for both v0.95 and > v1.0 drivers, with some slight aid from QEMU for > detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the > former we still rely on vendor for a performant data vqs implementation, > leaving the question to what it may end up eventually in the kernel is > effectively the latter). > > Thanks, > -SiweiMy suggestion is post the kernel patches, and we can evaluate how much work they are.> > > > Thanks > > > > > > > > > Meanwhile, I'll check internally to see if a legacy only model would > > > work. Thanks. > > > > > > Thanks, > > > -Siwei > > > > > > > > > > > > > > > > > On 3/2/2021 2:53 AM, Jason Wang wrote: > > > > > > > > On 2021/3/2 5:47 ??, Michael S. Tsirkin wrote: > > > > > > > > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote: > > > > > > > > > > On 2021/3/1 5:34 ??, Michael S. Tsirkin wrote: > > > > > > > > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote: > > > > > > > > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify > > > > > > > > > > > > > the kernel that it's a legacy guest. Ugh :( > > > > > > > > > > > > Well, although I think adding an ioctl is doable, may I > > > > > > > > > > > > know what the use > > > > > > > > > > > > case there will be for kernel to leverage such info > > > > > > > > > > > > directly? Is there a > > > > > > > > > > > > case QEMU can't do with dedicate ioctls later if there's indeed > > > > > > > > > > > > differentiation (legacy v.s. modern) needed? > > > > > > > > > > > BTW a good API could be > > > > > > > > > > > > > > > > > > > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int) > > > > > > > > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int) > > > > > > > > > > > > > > > > > > > > > > we did it per vring but maybe that was a mistake ... > > > > > > > > > > Actually, I wonder whether it's good time to just not support > > > > > > > > > > legacy driver > > > > > > > > > > for vDPA. Consider: > > > > > > > > > > > > > > > > > > > > 1) It's definition is no-normative > > > > > > > > > > 2) A lot of budren of codes > > > > > > > > > > > > > > > > > > > > So qemu can still present the legacy device since the config > > > > > > > > > > space or other > > > > > > > > > > stuffs that is presented by vhost-vDPA is not expected to be > > > > > > > > > > accessed by > > > > > > > > > > guest directly. Qemu can do the endian conversion when necessary > > > > > > > > > > in this > > > > > > > > > > case? > > > > > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > Overall I would be fine with this approach but we need to avoid breaking > > > > > > > > > working userspace, qemu releases with vdpa support are out there and > > > > > > > > > seem to work for people. Any changes need to take that into account > > > > > > > > > and document compatibility concerns. > > > > > > > > Agree, let me check. > > > > > > > > > > > > > > > > > > > > > > > > > I note that any hardware > > > > > > > > > implementation is already broken for legacy except on platforms with > > > > > > > > > strong ordering which might be helpful in reducing the scope. > > > > > > > > Yes. > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > >
Si-Wei Liu
2021-Dec-16 02:01 UTC
vdpa legacy guest support (was Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero)
On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote:> On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote: >> >> On 12/14/2021 6:06 PM, Jason Wang wrote: >>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <si-wei.liu at oracle.com> wrote: >>>> >>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote: >>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote: >>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote: >>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote: >>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion >>>>>>>> it ended up with. I have the following questions, >>>>>>>> >>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the >>>>>>>> support will be completely dropped from the table, is my understanding >>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86, >>>>>>>> which is backed by the spec at >>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure >>>>>>>> if there's request/need to support wilder legacy virtio versions earlier >>>>>>>> beyond. >>>>>>> I personally feel it's less work to add in kernel than try to >>>>>>> work around it in userspace. Jason feels differently. >>>>>>> Maybe post the patches and this will prove to Jason it's not >>>>>>> too terrible? >>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring >>>>>> layout level and is limited to x86 only, there should be easy way out. >>>>> Note a subtle difference: what matters is that guest, not host is x86. >>>>> Matters for emulators which might reorder memory accesses. >>>>> I guess this enforcement belongs in QEMU then? >>>> Right, I mean to get started, the initial guest driver support and the >>>> corresponding QEMU support for transitional vdpa backend can be limited >>>> to x86 guest/host only. Since the config space is emulated in QEMU, I >>>> suppose it's not hard to enforce in QEMU. >>> It's more than just config space, most devices have headers before the buffer. >> The ordering in datapath (data VQs) would have to rely on vendor's support. >> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays >> can/should well support the case when ORDER_PLATFORM is not acked by the >> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver >> today), even with v1.0 spec conforming and modern only vDPA device. The >> control VQ is implemented in software in the kernel, which can be easily >> accommodated/fixed when needed. >> >>>> QEMU can drive GET_LEGACY, >>>> GET_ENDIAN et al ioctls in advance to get the capability from the >>>> individual vendor driver. For that, we need another negotiation protocol >>>> similar to vhost_user's protocol_features between the vdpa kernel and >>>> QEMU, way before the guest driver is ever probed and its feature >>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call >>>> from the device, but we can assume weak ordering for legacy at this >>>> point (x86 only)? >>> I'm lost here, we have get_features() so: >> I assume here you refer to get_device_features() that Eli just changed the >> name. >>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie >>> 2) ORDER_PLATFORM means device requires platform ordering >>> >>> Any reason for having a new API for this? >> Are you going to enforce all vDPA hardware vendors to support the >> transitional model for legacy guest? meaning guest not acknowledging >> VERSION_1 would use the legacy interfaces captured in the spec section 7.4 >> (regarding ring layout, native endianness, message framing, vq alignment of >> 4096, 32bit feature, no features_ok bit in status, IO port interface i.e. >> all the things) instead? Noted we don't yet have a set_device_features() >> that allows the vdpa device to tell whether it is operating in transitional >> or modern-only mode. For software virtio, all support for the legacy part in >> a transitional model has been built up there already, however, it's not easy >> for vDPA vendors to implement all the requirements for an all-or-nothing >> legacy guest support (big endian guest for example). To these vendors, the >> legacy support within a transitional model is more of feature to them and >> it's best to leave some flexibility for them to implement partial support >> for legacy. That in turn calls out the need for a vhost-user protocol >> feature like negotiation API that can prohibit those unsupported guest >> setups to as early as backend_init before launching the VM. > Right. Of note is the fact that it's a spec bug which I > hope yet to fix, though due to existing guest code the > fix won't be complete.I thought at one point you pointed out to me that the spec does allow config space read before claiming features_ok, and only config write before features_ok is prohibited. I haven't read up the full thread of Halil's VERSION_1 for transitional big endian device yet, but what is the spec bug you hope to fix?> > WRT ioctls, One thing we can do though is abuse set_features > where it's called by QEMU early on with just the VERSION_1 > bit set, to distinguish between legacy and modern > interface. This before config space accesses and FEATURES_OK. > > Halil has been working on this, pls take a look and maybe help him out.Interesting thread, am reading now and see how I may leverage or help there.>>>>>> I >>>>>> checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level >>>>>> 0.95 support, it seems all the ingredient had been there already dated back >>>>>> to the DPDK days. The only major thing limiting is in the vDPA software that >>>>>> the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for >>>>>> a few DMA setup ops, which is virtio 1.0 only. >>>>>> >>>>>>>> 2. suppose some form of legacy guest support needs to be there, how do we >>>>>>>> deal with the bogus assumption below in vdpa_get_config() in the short term? >>>>>>>> It looks one of the intuitive fix is to move the vdpa_set_features call out >>>>>>>> of vdpa_get_config() to vdpa_set_config(). >>>>>>>> >>>>>>>> /* >>>>>>>> * Config accesses aren't supposed to trigger before features are >>>>>>>> set. >>>>>>>> * If it does happen we assume a legacy guest. >>>>>>>> */ >>>>>>>> if (!vdev->features_valid) >>>>>>>> vdpa_set_features(vdev, 0); >>>>>>>> ops->get_config(vdev, offset, buf, len); >>>>>>>> >>>>>>>> I can post a patch to fix 2) if there's consensus already reached. >>>>>>>> >>>>>>>> Thanks, >>>>>>>> -Siwei >>>>>>> I'm not sure how important it is to change that. >>>>>>> In any case it only affects transitional devices, right? >>>>>>> Legacy only should not care ... >>>>>> Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the >>>>>> modern one in a transitional device model rather than being legacy only. >>>>>> That way a v0.95 and v1.0 supporting vdpa parent can support both types of >>>>>> guests without having to reconfigure. Or are you suggesting limit to legacy >>>>>> only at the time of vdpa creation would simplify the implementation a lot? >>>>>> >>>>>> Thanks, >>>>>> -Siwei >>>>> I don't know for sure. Take a look at the work Halil was doing >>>>> to try and support transitional devices with BE guests. >>>> Hmmm, we can have those endianness ioctls defined but the initial QEMU >>>> implementation can be started to support x86 guest/host with little >>>> endian and weak memory ordering first. The real trick is to detect >>>> legacy guest - I am not sure if it's feasible to shift all the legacy >>>> detection work to QEMU, or the kernel has to be part of the detection >>>> (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking >>>> effort in QEMU) as well. Let me take a further look and get back. >>> Michael may think differently but I think doing this in Qemu is much easier. >> I think the key is whether we position emulating legacy interfaces in QEMU >> doing translation on top of a v1.0 modern-only device in the kernel, or we >> allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a >> transitional model in the kernel that is able to work for both v0.95 and >> v1.0 drivers, with some slight aid from QEMU for >> detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the >> former we still rely on vendor for a performant data vqs implementation, >> leaving the question to what it may end up eventually in the kernel is >> effectively the latter). >> >> Thanks, >> -Siwei > > My suggestion is post the kernel patches, and we can evaluate > how much work they are.Thanks for the feedback. I will take some read then get back, probably after the winter break. Stay tuned. Thanks, -Siwei> >>> Thanks >>> >>> >>> >>>> Meanwhile, I'll check internally to see if a legacy only model would >>>> work. Thanks. >>>> >>>> Thanks, >>>> -Siwei >>>> >>>> >>>>>>>> On 3/2/2021 2:53 AM, Jason Wang wrote: >>>>>>>>> On 2021/3/2 5:47 ??, Michael S. Tsirkin wrote: >>>>>>>>>> On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote: >>>>>>>>>>> On 2021/3/1 5:34 ??, Michael S. Tsirkin wrote: >>>>>>>>>>>> On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote: >>>>>>>>>>>>>> Detecting it isn't enough though, we will need a new ioctl to notify >>>>>>>>>>>>>> the kernel that it's a legacy guest. Ugh :( >>>>>>>>>>>>> Well, although I think adding an ioctl is doable, may I >>>>>>>>>>>>> know what the use >>>>>>>>>>>>> case there will be for kernel to leverage such info >>>>>>>>>>>>> directly? Is there a >>>>>>>>>>>>> case QEMU can't do with dedicate ioctls later if there's indeed >>>>>>>>>>>>> differentiation (legacy v.s. modern) needed? >>>>>>>>>>>> BTW a good API could be >>>>>>>>>>>> >>>>>>>>>>>> #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int) >>>>>>>>>>>> #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int) >>>>>>>>>>>> >>>>>>>>>>>> we did it per vring but maybe that was a mistake ... >>>>>>>>>>> Actually, I wonder whether it's good time to just not support >>>>>>>>>>> legacy driver >>>>>>>>>>> for vDPA. Consider: >>>>>>>>>>> >>>>>>>>>>> 1) It's definition is no-normative >>>>>>>>>>> 2) A lot of budren of codes >>>>>>>>>>> >>>>>>>>>>> So qemu can still present the legacy device since the config >>>>>>>>>>> space or other >>>>>>>>>>> stuffs that is presented by vhost-vDPA is not expected to be >>>>>>>>>>> accessed by >>>>>>>>>>> guest directly. Qemu can do the endian conversion when necessary >>>>>>>>>>> in this >>>>>>>>>>> case? >>>>>>>>>>> >>>>>>>>>>> Thanks >>>>>>>>>>> >>>>>>>>>> Overall I would be fine with this approach but we need to avoid breaking >>>>>>>>>> working userspace, qemu releases with vdpa support are out there and >>>>>>>>>> seem to work for people. Any changes need to take that into account >>>>>>>>>> and document compatibility concerns. >>>>>>>>> Agree, let me check. >>>>>>>>> >>>>>>>>> >>>>>>>>>> I note that any hardware >>>>>>>>>> implementation is already broken for legacy except on platforms with >>>>>>>>>> strong ordering which might be helpful in reducing the scope. >>>>>>>>> Yes. >>>>>>>>> >>>>>>>>> Thanks >>>>>>>>> >>>>>>>>>