Linux Virtualization - Apr 2015 - [dpdk-dev] [snabb-devel] Re: memory barriers in virtq.lua?

Hi Michael,

I'm writing to follow up the previous discussion about memory barriers in
virtio-net device implementations, and Cc'ing the DPDK list because I
believe this is relevant to them too.

First, thanks again for getting in touch and reviewing our code.

I have now found a missed case where we *do* require a hardware memory
barrier on x86 in our vhost/virtio-net device. That is when checking the
interrupt suppression flag after updating used->idx. This is needed because
x86 can reorder the write to used->idx after the read from avail->flags,
and that causes the guest to see a stale value of used->idx after it
toggles interrupt suppression.

If I may spell out my mental model, for the sake of being corrected and/or
as an example of how third party developers are reading and interpreting
the Virtio-net spec:

Relating this to Virtio 1.0, the most relevant section is 3.2.1 (Supplying
Buffers to the Device) which calls for two "suitable memory barriers".
The
spec talks about these from the driver perspective, but they are both
relevant to the device side too.

The first barrier (write to descriptor table before write to used->idx) is
implicit on x86 because writes by the same core are not reordered. This
means that no explicit hardware barrier is needed. (A compiler barrier may
be needed, however.)

The second memory barrier (write to used->idx before reading avail->flags)
is not implicit on x86 because stores are reordered after loads. So an
explicit hardware memory barrier is needed.

I hope that is a correct assessment of the situation. (Forgive my
x86centricity, I am sure that seems very foreign to kernel hackers.)

If this assessment is correct then the DPDK developers might also want to
review librte_vhost/vhost_rxtx.c and consider adding a hardware memory
barrier between writing used->idx and reading avail->flags.

Cheers,
-Luke

P.S. I notice that the Linux virtio-net driver does not seem to tolerate
spurious interrupts, even though the Virtio 1.0 spec requires this
("must"). On 3.13.11-ckt15 I see them trigger an "irq nobody
cared" kernel
log message and then the irq is disabled. If that sounds suspicious I can
supply more information.
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On Tue, Apr 07, 2015 at 04:22:42PM +0200, Luke Gorrie
wrote:
> Hi Michael,
> 
> I'm writing to follow up the previous discussion about memory barriers
in
> virtio-net device implementations, and Cc'ing the DPDK list because I
believe
> this is relevant to them too.
> 
> First, thanks again for getting in touch and reviewing our code.
> 
> I have now found a missed case where we *do* require a hardware memory
barrier
> on x86 in our vhost/virtio-net device. That is when checking the interrupt
> suppression flag after updating used->idx. This is needed because x86
can
> reorder the write to used->idx after the read from avail->flags, and
that
> causes the guest to see a stale value of used->idx after it toggles
interrupt
> suppression.
> 
> If I may spell out my mental model, for the sake of being corrected and/or
as
> an example of how third party developers are reading and interpreting the
> Virtio-net spec:
> 
> Relating this to Virtio 1.0, the most relevant section is 3.2.1 (Supplying
> Buffers to the Device) which calls for two "suitable memory
barriers". The spec
> talks about these from the driver perspective, but they are both relevant
to
> the device side too.
> 
> The first barrier (write to descriptor table before write to used->idx)
is
> implicit on x86 because writes by the same core are not reordered. This
means
> that no explicit hardware barrier is needed. (A compiler barrier may be
needed,
> however.)
> 
> The second memory barrier (write to used->idx before reading
avail->flags) is
> not implicit on x86 because stores are reordered after loads. So an
explicit
> hardware memory barrier is needed.
> 
> I hope that is a correct assessment of the situation. (Forgive my
> x86centricity, I am sure that seems very foreign to kernel hackers.)
> 
> If this assessment is correct then the DPDK developers might also want to
> review librte_vhost/vhost_rxtx.c and consider adding a hardware memory
barrier
> between writing used->idx and reading avail->flags.
> 
> Cheers,
> -Luke
I agree, this looks like a bug in dpdk.
> P.S. I notice that the Linux virtio-net driver does not seem to tolerate
> spurious interrupts, even though the Virtio 1.0 spec requires this
("must"). On
> 3.13.11-ckt15 I see them trigger an "irq nobody cared" kernel log
message and
> then the irq is disabled. If that sounds suspicious I can supply more
> information.
> 
>
More information might be useful, yes.

Just guessing from the available info:

I think you refer to this:
        The driver MUST handle spurious interrupts from the device.

The intent is to be able to handle some spurious interrupts once in a
while.  AFAIK linux triggers the message if it gets a huge number of
spurious interrupts for an extended period of time.
For example, this will trigger if the device does not clear interrupt
line after interrupt register read.

On 7 April 2015 at 17:30, Michael S. Tsirkin <mst at redhat.com> wrote:
> Just guessing from the available info:
>
> I think you refer to this:
>         The driver MUST handle spurious interrupts from the device.
>
> The intent is to be able to handle some spurious interrupts once in a
> while.  AFAIK linux triggers the message if it gets a huge number of
> spurious interrupts for an extended period of time.
> For example, this will trigger if the device does not clear interrupt
> line after interrupt register read.
>
Thanks for that info.

The only spurious interrupt that I think we need is one when vhost-user
reconnects. That would be to cover the case where the vswitch is restarted
after writing used->idx but before sending the interrupt.

Or perhaps there is a better solution to that case?

Looking forward to getting an upstream vhost-user reconnect. one thing at a
time.. :)

Cheers,
-Luke
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On 4/7/2015 10:23 PM, Luke Gorrie wrote:
> Hi Michael,
>
> I'm writing to follow up the previous discussion about memory barriers
in
> virtio-net device implementations, and Cc'ing the DPDK list because I
> believe this is relevant to them too.
>
> First, thanks again for getting in touch and reviewing our code.
>
> I have now found a missed case where we *do* require a hardware memory
> barrier on x86 in our vhost/virtio-net device. That is when checking the
> interrupt suppression flag after updating used->idx. This is needed
because
> x86 can reorder the write to used->idx after the read from
avail->flags,
> and that causes the guest to see a stale value of used->idx after it
> toggles interrupt suppression.
luke:
1. host read the flag. 2 guest toggles the flag 3.guest checks the used.
4. host update used.
Is this your case?
>
> If I may spell out my mental model, for the sake of being corrected and/or
> as an example of how third party developers are reading and interpreting
> the Virtio-net spec:
>
> Relating this to Virtio 1.0, the most relevant section is 3.2.1 (Supplying
> Buffers to the Device) which calls for two "suitable memory
barriers". The
> spec talks about these from the driver perspective, but they are both
> relevant to the device side too.
>
> The first barrier (write to descriptor table before write to used->idx)
is
> implicit on x86 because writes by the same core are not reordered. This
> means that no explicit hardware barrier is needed. (A compiler barrier may
> be needed, however.)
>
> The second memory barrier (write to used->idx before reading
avail->flags)
> is not implicit on x86 because stores are reordered after loads. So an
> explicit hardware memory barrier is needed.
>
> I hope that is a correct assessment of the situation. (Forgive my
> x86centricity, I am sure that seems very foreign to kernel hackers.)
>
> If this assessment is correct then the DPDK developers might also want to
> review librte_vhost/vhost_rxtx.c and consider adding a hardware memory
> barrier between writing used->idx and reading avail->flags.
>
> Cheers,
> -Luke
>
> P.S. I notice that the Linux virtio-net driver does not seem to tolerate
> spurious interrupts, even though the Virtio 1.0 spec requires this
> ("must"). On 3.13.11-ckt15 I see them trigger an "irq nobody
cared" kernel
> log message and then the irq is disabled. If that sounds suspicious I can
> supply more information.
>
>

Howdy,

On 8 April 2015 at 17:15, Xie, Huawei <huawei.xie at intel.com> wrote:
> luke:
> 1. host read the flag. 2 guest toggles the flag 3.guest checks the used.
> 4. host update used.
> Is this your case?
>
Yep, that is exactly the case I mean.

Cheers,
-Luke
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