Linux Virtualization - Apr 2019 - [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

Michael S. Tsirkin <mst at redhat.com> writes:
> On Wed, Mar 20, 2019 at 01:13:41PM -0300, Thiago Jung Bauermann wrote:
>> >> Another way of looking at this issue which also explains our
reluctance
>> >> is that the only difference between a secure guest and a
regular guest
>> >> (at least regarding virtio) is that the former uses swiotlb
while the
>> >> latter doens't.
>> >
>> > But swiotlb is just one implementation. It's a guest internal
thing. The
>> > issue is that memory isn't host accessible.
>>
>> >From what I understand of the ACCESS_PLATFORM definition, the host
will
>> only ever try to access memory addresses that are supplied to it by the
>> guest, so all of the secure guest memory that the host cares about is
>> accessible:
>>
>>     If this feature bit is set to 0, then the device has same access to
>>     memory addresses supplied to it as the driver has. In particular,
>>     the device will always use physical addresses matching addresses
>>     used by the driver (typically meaning physical addresses used by
the
>>     CPU) and not translated further, and can access any address
supplied
>>     to it by the driver. When clear, this overrides any
>>     platform-specific description of whether device access is limited
or
>>     translated in any way, e.g. whether an IOMMU may be present.
>>
>> All of the above is true for POWER guests, whether they are secure
>> guests or not.
>>
>> Or are you saying that a virtio device may want to access memory
>> addresses that weren't supplied to it by the driver?
>
> Your logic would apply to IOMMUs as well.  For your mode, there are
> specific encrypted memory regions that driver has access to but device
> does not. that seems to violate the constraint.
Right, if there's a pre-configured 1:1 mapping in the IOMMU such that
the device can ignore the IOMMU for all practical purposes I would
indeed say that the logic would apply to IOMMUs as well. :-)

I guess I'm still struggling with the purpose of signalling to the
driver that the host may not have access to memory addresses that it
will never try to access.
>> >> And from the device's point of view they're
>> >> indistinguishable. It can't tell one guest that is using
swiotlb from
>> >> one that isn't. And that implies that secure guest vs
regular guest
>> >> isn't a virtio interface issue, it's "guest
internal affairs". So
>> >> there's no reason to reflect that in the feature flags.
>> >
>> > So don't. The way not to reflect that in the feature flags is
>> > to set ACCESS_PLATFORM.  Then you say *I don't care let
platform device*.
>> >
>> >
>> > Without ACCESS_PLATFORM
>> > virtio has a very specific opinion about the security of the
>> > device, and that opinion is that device is part of the guest
>> > supervisor security domain.
>>
>> Sorry for being a bit dense, but not sure what "the device is part
of
>> the guest supervisor security domain" means. In powerpc-speak,
>> "supervisor" is the operating system so perhaps that explains
my
>> confusion. Are you saying that without ACCESS_PLATFORM, the guest
>> considers the host to be part of the guest operating system's
security
>> domain?
>
> I think so. The spec says "device has same access as driver".
Ok, makes sense.
>> If so, does that have any other implication besides "the host
>> can access any address supplied to it by the driver"? If that is
the
>> case, perhaps the definition of ACCESS_PLATFORM needs to be amended to
>> include that information because it's not part of the current
>> definition.
>>
>> >> > But the name "sev_active" makes me scared
because at least AMD guys who
>> >> > were doing the sensible thing and setting ACCESS_PLATFORM
>> >>
>> >> My understanding is, AMD guest-platform knows in advance that
their
>> >> guest will run in secure mode and hence sets the flag at the
time of VM
>> >> instantiation. Unfortunately we dont have that luxury on our
platforms.
>> >
>> > Well you do have that luxury. It looks like that there are
existing
>> > guests that already acknowledge ACCESS_PLATFORM and you are not
happy
>> > with how that path is slow. So you are trying to optimize for
>> > them by clearing ACCESS_PLATFORM and then you have lost ability
>> > to invoke DMA API.
>> >
>> > For example if there was another flag just like ACCESS_PLATFORM
>> > just not yet used by anyone, you would be all fine using that
right?
>>
>> Yes, a new flag sounds like a great idea. What about the definition
>> below?
>>
>> VIRTIO_F_ACCESS_PLATFORM_NO_IOMMU This feature has the same meaning as
>>     VIRTIO_F_ACCESS_PLATFORM both when set and when not set, with the
>>     exception that the IOMMU is explicitly defined to be off or
bypassed
>>     when accessing memory addresses supplied to the device by the
>>     driver. This flag should be set by the guest if offered, but to
>>     allow for backward-compatibility device implementations allow for
it
>>     to be left unset by the guest. It is an error to set both this flag
>>     and VIRTIO_F_ACCESS_PLATFORM.
>
> It looks kind of narrow but it's an option.
Great!
> I wonder how we'll define what's an iommu though.
Hm, it didn't occur to me it could be an issue. I'll try.
> Another idea is maybe something like virtio-iommu?
You mean, have legacy guests use virtio-iommu to request an IOMMU
bypass? If so, it's an interesting idea for new guests but it doesn't
help with guests that are out today in the field, which don't have A
virtio-iommu driver.
>> > Is there any justification to doing that beyond someone putting
>> > out slow code in the past?
>>
>> The definition of the ACCESS_PLATFORM flag is generic and captures the
>> notion of memory access restrictions for the device. Unfortunately, on
>> powerpc pSeries guests it also implies that the IOMMU is turned on
>
> IIUC that's really because on pSeries IOMMU is *always* turned on.
> Platform has no way to say what you want it to say
> which is bypass the iommu for the specific device.
Yes, that's correct. pSeries guests running on KVM are in a gray area
where theoretically they use an IOMMU but in practice KVM ignores it.
It's unfortunate but it's the reality on the ground today. :-/

--
Thiago Jung Bauermann
IBM Linux Technology Center

On Thu, Mar 21, 2019 at 09:05:04PM -0300, Thiago Jung Bauermann
wrote:
> 
> Michael S. Tsirkin <mst at redhat.com> writes:
> 
> > On Wed, Mar 20, 2019 at 01:13:41PM -0300, Thiago Jung Bauermann wrote:
> >> >> Another way of looking at this issue which also explains
our reluctance
> >> >> is that the only difference between a secure guest and a
regular guest
> >> >> (at least regarding virtio) is that the former uses
swiotlb while the
> >> >> latter doens't.
> >> >
> >> > But swiotlb is just one implementation. It's a guest
internal thing. The
> >> > issue is that memory isn't host accessible.
> >>
> >> >From what I understand of the ACCESS_PLATFORM definition, the
host will
> >> only ever try to access memory addresses that are supplied to it
by the
> >> guest, so all of the secure guest memory that the host cares about
is
> >> accessible:
> >>
> >>     If this feature bit is set to 0, then the device has same
access to
> >>     memory addresses supplied to it as the driver has. In
particular,
> >>     the device will always use physical addresses matching
addresses
> >>     used by the driver (typically meaning physical addresses used
by the
> >>     CPU) and not translated further, and can access any address
supplied
> >>     to it by the driver. When clear, this overrides any
> >>     platform-specific description of whether device access is
limited or
> >>     translated in any way, e.g. whether an IOMMU may be present.
> >>
> >> All of the above is true for POWER guests, whether they are secure
> >> guests or not.
> >>
> >> Or are you saying that a virtio device may want to access memory
> >> addresses that weren't supplied to it by the driver?
> >
> > Your logic would apply to IOMMUs as well.  For your mode, there are
> > specific encrypted memory regions that driver has access to but device
> > does not. that seems to violate the constraint.
> 
> Right, if there's a pre-configured 1:1 mapping in the IOMMU such that
> the device can ignore the IOMMU for all practical purposes I would
> indeed say that the logic would apply to IOMMUs as well. :-)
> 
> I guess I'm still struggling with the purpose of signalling to the
> driver that the host may not have access to memory addresses that it
> will never try to access.
For example, one of the benefits is to signal to host that driver does
not expect ability to access all memory. If it does, host can
fail initialization gracefully.
> >> >> And from the device's point of view they're
> >> >> indistinguishable. It can't tell one guest that is
using swiotlb from
> >> >> one that isn't. And that implies that secure guest vs
regular guest
> >> >> isn't a virtio interface issue, it's "guest
internal affairs". So
> >> >> there's no reason to reflect that in the feature
flags.
> >> >
> >> > So don't. The way not to reflect that in the feature
flags is
> >> > to set ACCESS_PLATFORM.  Then you say *I don't care let
platform device*.
> >> >
> >> >
> >> > Without ACCESS_PLATFORM
> >> > virtio has a very specific opinion about the security of the
> >> > device, and that opinion is that device is part of the guest
> >> > supervisor security domain.
> >>
> >> Sorry for being a bit dense, but not sure what "the device is
part of
> >> the guest supervisor security domain" means. In
powerpc-speak,
> >> "supervisor" is the operating system so perhaps that
explains my
> >> confusion. Are you saying that without ACCESS_PLATFORM, the guest
> >> considers the host to be part of the guest operating system's
security
> >> domain?
> >
> > I think so. The spec says "device has same access as
driver".
> 
> Ok, makes sense.
> 
> >> If so, does that have any other implication besides "the host
> >> can access any address supplied to it by the driver"? If that
is the
> >> case, perhaps the definition of ACCESS_PLATFORM needs to be
amended to
> >> include that information because it's not part of the current
> >> definition.
> >>
> >> >> > But the name "sev_active" makes me scared
because at least AMD guys who
> >> >> > were doing the sensible thing and setting
ACCESS_PLATFORM
> >> >>
> >> >> My understanding is, AMD guest-platform knows in advance
that their
> >> >> guest will run in secure mode and hence sets the flag at
the time of VM
> >> >> instantiation. Unfortunately we dont have that luxury on
our platforms.
> >> >
> >> > Well you do have that luxury. It looks like that there are
existing
> >> > guests that already acknowledge ACCESS_PLATFORM and you are
not happy
> >> > with how that path is slow. So you are trying to optimize for
> >> > them by clearing ACCESS_PLATFORM and then you have lost
ability
> >> > to invoke DMA API.
> >> >
> >> > For example if there was another flag just like
ACCESS_PLATFORM
> >> > just not yet used by anyone, you would be all fine using that
right?
> >>
> >> Yes, a new flag sounds like a great idea. What about the
definition
> >> below?
> >>
> >> VIRTIO_F_ACCESS_PLATFORM_NO_IOMMU This feature has the same
meaning as
> >>     VIRTIO_F_ACCESS_PLATFORM both when set and when not set, with
the
> >>     exception that the IOMMU is explicitly defined to be off or
bypassed
> >>     when accessing memory addresses supplied to the device by the
> >>     driver. This flag should be set by the guest if offered, but
to
> >>     allow for backward-compatibility device implementations allow
for it
> >>     to be left unset by the guest. It is an error to set both this
flag
> >>     and VIRTIO_F_ACCESS_PLATFORM.
> >
> > It looks kind of narrow but it's an option.
> 
> Great!
> 
> > I wonder how we'll define what's an iommu though.
> 
> Hm, it didn't occur to me it could be an issue. I'll try.
> 
> > Another idea is maybe something like virtio-iommu?
> 
> You mean, have legacy guests use virtio-iommu to request an IOMMU
> bypass? If so, it's an interesting idea for new guests but it
doesn't
> help with guests that are out today in the field, which don't have A
> virtio-iommu driver.
I presume legacy guests don't use encrypted memory so why do we
worry about them at all?
> >> > Is there any justification to doing that beyond someone
putting
> >> > out slow code in the past?
> >>
> >> The definition of the ACCESS_PLATFORM flag is generic and captures
the
> >> notion of memory access restrictions for the device.
Unfortunately, on
> >> powerpc pSeries guests it also implies that the IOMMU is turned on
> >
> > IIUC that's really because on pSeries IOMMU is *always* turned on.
> > Platform has no way to say what you want it to say
> > which is bypass the iommu for the specific device.
> 
> Yes, that's correct. pSeries guests running on KVM are in a gray area
> where theoretically they use an IOMMU but in practice KVM ignores it.
> It's unfortunate but it's the reality on the ground today. :-/
Well it's not just the reality, virt setups need something that
emulated IOMMUs don't provide. That is not uncommon, e.g.
intel's VTD has a "cache mode" field which AFAIK is only used for
virt.
> --
> Thiago Jung Bauermann
> IBM Linux Technology Center

On Sat, Mar 23, 2019 at 05:01:35PM -0400, Michael S. Tsirkin
wrote:
> On Thu, Mar 21, 2019 at 09:05:04PM -0300, Thiago Jung Bauermann wrote:
> > Michael S. Tsirkin <mst at redhat.com> writes:
[snip]
> > >> > Is there any justification to doing that beyond someone
putting
> > >> > out slow code in the past?
> > >>
> > >> The definition of the ACCESS_PLATFORM flag is generic and
captures the
> > >> notion of memory access restrictions for the device.
Unfortunately, on
> > >> powerpc pSeries guests it also implies that the IOMMU is
turned on
> > >
> > > IIUC that's really because on pSeries IOMMU is *always*
turned on.
> > > Platform has no way to say what you want it to say
> > > which is bypass the iommu for the specific device.
> > 
> > Yes, that's correct. pSeries guests running on KVM are in a gray
area
> > where theoretically they use an IOMMU but in practice KVM ignores it.
> > It's unfortunate but it's the reality on the ground today. :-/
Um.. I'm not sure what you mean by this.  As far as I'm concerned
there is always a guest-visible (paravirtualized) IOMMU, and that will
be backed onto the host IOMMU when necessary.

[Actually there is an IOMMU bypass hack that's used by the guest
 firmware, but I don't think we want to expose that]
> Well it's not just the reality, virt setups need something that
> emulated IOMMUs don't provide. That is not uncommon, e.g.
> intel's VTD has a "cache mode" field which AFAIK is only used
for virt.
-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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Michael S. Tsirkin <mst at redhat.com> writes:
> On Thu, Mar 21, 2019 at 09:05:04PM -0300, Thiago Jung Bauermann wrote:
>>
>> Michael S. Tsirkin <mst at redhat.com> writes:
>>
>> > On Wed, Mar 20, 2019 at 01:13:41PM -0300, Thiago Jung Bauermann
wrote:
>> >> >From what I understand of the ACCESS_PLATFORM definition,
the host will
>> >> only ever try to access memory addresses that are supplied to
it by the
>> >> guest, so all of the secure guest memory that the host cares
about is
>> >> accessible:
>> >>
>> >>     If this feature bit is set to 0, then the device has same
access to
>> >>     memory addresses supplied to it as the driver has. In
particular,
>> >>     the device will always use physical addresses matching
addresses
>> >>     used by the driver (typically meaning physical addresses
used by the
>> >>     CPU) and not translated further, and can access any
address supplied
>> >>     to it by the driver. When clear, this overrides any
>> >>     platform-specific description of whether device access is
limited or
>> >>     translated in any way, e.g. whether an IOMMU may be
present.
>> >>
>> >> All of the above is true for POWER guests, whether they are
secure
>> >> guests or not.
>> >>
>> >> Or are you saying that a virtio device may want to access
memory
>> >> addresses that weren't supplied to it by the driver?
>> >
>> > Your logic would apply to IOMMUs as well.  For your mode, there
are
>> > specific encrypted memory regions that driver has access to but
device
>> > does not. that seems to violate the constraint.
>>
>> Right, if there's a pre-configured 1:1 mapping in the IOMMU such
that
>> the device can ignore the IOMMU for all practical purposes I would
>> indeed say that the logic would apply to IOMMUs as well. :-)
>>
>> I guess I'm still struggling with the purpose of signalling to the
>> driver that the host may not have access to memory addresses that it
>> will never try to access.
>
> For example, one of the benefits is to signal to host that driver does
> not expect ability to access all memory. If it does, host can
> fail initialization gracefully.
But why would the ability to access all memory be necessary or even
useful? When would the host access memory that the driver didn't tell it
to access?
>> >> >> > But the name "sev_active" makes me
scared because at least AMD guys who
>> >> >> > were doing the sensible thing and setting
ACCESS_PLATFORM
>> >> >>
>> >> >> My understanding is, AMD guest-platform knows in
advance that their
>> >> >> guest will run in secure mode and hence sets the flag
at the time of VM
>> >> >> instantiation. Unfortunately we dont have that luxury
on our platforms.
>> >> >
>> >> > Well you do have that luxury. It looks like that there
are existing
>> >> > guests that already acknowledge ACCESS_PLATFORM and you
are not happy
>> >> > with how that path is slow. So you are trying to optimize
for
>> >> > them by clearing ACCESS_PLATFORM and then you have lost
ability
>> >> > to invoke DMA API.
>> >> >
>> >> > For example if there was another flag just like
ACCESS_PLATFORM
>> >> > just not yet used by anyone, you would be all fine using
that right?
>> >>
>> >> Yes, a new flag sounds like a great idea. What about the
definition
>> >> below?
>> >>
>> >> VIRTIO_F_ACCESS_PLATFORM_NO_IOMMU This feature has the same
meaning as
>> >>     VIRTIO_F_ACCESS_PLATFORM both when set and when not set,
with the
>> >>     exception that the IOMMU is explicitly defined to be off
or bypassed
>> >>     when accessing memory addresses supplied to the device by
the
>> >>     driver. This flag should be set by the guest if offered,
but to
>> >>     allow for backward-compatibility device implementations
allow for it
>> >>     to be left unset by the guest. It is an error to set both
this flag
>> >>     and VIRTIO_F_ACCESS_PLATFORM.
>> >
>> > It looks kind of narrow but it's an option.
>>
>> Great!
>>
>> > I wonder how we'll define what's an iommu though.
>>
>> Hm, it didn't occur to me it could be an issue. I'll try.
I rephrased it in terms of address translation. What do you think of
this version? The flag name is slightly different too:


VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION This feature has the same
    meaning as VIRTIO_F_ACCESS_PLATFORM both when set and when not set,
    with the exception that address translation is guaranteed to be
    unnecessary when accessing memory addresses supplied to the device
    by the driver. Which is to say, the device will always use physical
    addresses matching addresses used by the driver (typically meaning
    physical addresses used by the CPU) and not translated further. This
    flag should be set by the guest if offered, but to allow for
    backward-compatibility device implementations allow for it to be
    left unset by the guest. It is an error to set both this flag and
    VIRTIO_F_ACCESS_PLATFORM.
>> > Another idea is maybe something like virtio-iommu?
>>
>> You mean, have legacy guests use virtio-iommu to request an IOMMU
>> bypass? If so, it's an interesting idea for new guests but it
doesn't
>> help with guests that are out today in the field, which don't have
A
>> virtio-iommu driver.
>
> I presume legacy guests don't use encrypted memory so why do we
> worry about them at all?
They don't use encrypted memory, but a host machine will run a mix of
secure and legacy guests. And since the hypervisor doesn't know whether
a guest will be secure or not at the time it is launched, legacy guests
will have to be launched with the same configuration as secure guests.
>> >> > Is there any justification to doing that beyond someone
putting
>> >> > out slow code in the past?
>> >>
>> >> The definition of the ACCESS_PLATFORM flag is generic and
captures the
>> >> notion of memory access restrictions for the device.
Unfortunately, on
>> >> powerpc pSeries guests it also implies that the IOMMU is
turned on
>> >
>> > IIUC that's really because on pSeries IOMMU is *always* turned
on.
>> > Platform has no way to say what you want it to say
>> > which is bypass the iommu for the specific device.
>>
>> Yes, that's correct. pSeries guests running on KVM are in a gray
area
>> where theoretically they use an IOMMU but in practice KVM ignores it.
>> It's unfortunate but it's the reality on the ground today. :-/
>
> Well it's not just the reality, virt setups need something that
> emulated IOMMUs don't provide. That is not uncommon, e.g.
> intel's VTD has a "cache mode" field which AFAIK is only used
for virt.
That's good to know. Thanks for this example.

--
Thiago Jung Bauermann
IBM Linux Technology Center