Re: [PATCH v2 1/1] virtio: fix the condition for iommu_platform not supported

[Halil Pasic]

On Thu, 27 Jan 2022 18:34:23 -0300
Daniel Henrique Barboza <danielhb413@gmail.com> wrote:

> On 1/27/22 10:28, Halil Pasic wrote:
> > ping^2
> > 
> > Also adding Brijesh and Daniel, as I believe you guys should be
> > interested in this, and I'm yet to receive review.
> > 
> > @Brijesh, Daniel: Can you confirm that AMD (SEV) and Power are affected
> > too, and that the fix works for your platforms as well?  
> 
> I failed to find a host that has Power secure execution support. I'll keep 
> looking.
> 
> 
> Meanwhile I have to mention that this patch re-introduced the problem that 
> Kevin's
> commit fixed.
> 
> 
> With current upstream, if you start a regular guest with the following 
> command line:
> 
> qemu-system-ppc64 (....)
> -chardev socket,id=char0,path=/tmp/vhostqemu
> -device vhost-user-fs-pci,chardev=char0,tag=myfs,iommu_platform=on
> 
> i.e. a guest with a vhost-user-fs-pci device that claims to have iommu 
> support,
> but it doesn't, this is the error message:
> 
> 
> qemu-system-ppc64: -device 
> vhost-user-fs-pci,chardev=char0,tag=myfs,iommu_platform=on: 
> iommu_platform=true is not supported by the device
> 
> 
> With this patch, that command line above starts the guest. 
> virtiofsd fails during boot:
> 
> sudo ~/qemu/build/tools/virtiofsd/virtiofsd --socket-path=/tmp/vhostqemu -o 
> source=~/linux-L1
> [sudo] password for danielhb:
> virtio_session_mount: Waiting for vhost-user socket connection...
> virtio_session_mount: Received vhost-user socket connection
> virtio_loop: Entry
> fv_panic: libvhost-user: Invalid vring_addr message
> 
> 
> And inside the guest, if you attempt to mount and use the virtiofs 
> filesystem, the guest
> hangs:
> 
> [root@localhost ~]# mount -t virtiofs myfs /mnt
> [root@localhost ~]# cd /mnt
> 
> (hangs)
> 
> Exiting QEMU throws several vhost related errors:
> 
> 
> QEMU 6.2.50 monitor - type 'help' for more information
> (qemu) quit
> qemu-system-ppc64: Failed to set msg fds.
> qemu-system-ppc64: vhost VQ 0 ring restore failed: -22: Invalid argument (22)
> qemu-system-ppc64: Failed to set msg fds.
> qemu-system-ppc64: vhost VQ 1 ring restore failed: -22: Invalid argument (22)
> qemu-system-ppc64: Failed to set msg fds.
> qemu-system-ppc64: vhost_set_vring_call failed: Invalid argument (22)
> qemu-system-ppc64: Failed to set msg fds.
> qemu-system-ppc64: vhost_set_vring_call failed: Invalid argument (22)
> 
> 


Does your VM have an IOMMU and does your guest see it? If yes does
vdev->dma_as != &address_space_memory hold for your virtio device? If no why 
not?

My understanding is that your guest wants to do translated addresses,
because it sees the ACCESS_PLATFORM feature, and probably thinks that
your device is indeed behind an IOMMU, from what I assume, at least it
sees that there is an IOMMU. But then I would expect your virtio device
to have its vdev->dma_as set to something different than
&address_space_memory. Conversely if your dma address space is
address_space_memory, then you don't need address translation because
your dma addresses are the same  as your guest physical addresses.

> 
> I made a little experiment with upstream and reverting Kevin's patch and the 
> result is
> the same, meaning that this is the original bug [1] Kevin fixed back then. 
> Note that [1]
> was reported on x86, meaning that this particular issue seems to be arch 
> agnostic.

We don't have this problem on s390, so it ain't entirely arch agnostic.

> 
> 
> My point here is that your patch fixes the situation for s390x, and Brijesh 
> already chimed
> in claiming that it fixed for AMD SEV, but it reintroduced a bug. I believe 
> you should
> include this test case with vhost-user in your testing to figure out a way to 
> fix what
> is needed without adding this particular regression.

Can you help me with this? IMHO the big problem is that iommu_platform
is used for two distinct things. I've described that in the commit
message.

We may be able to differentiate between the two using ->dma_as, but for
that it needs to be set up correctly: whenever you require translation
it should be something different than address_space_memory. The question
is why do you require translation but don't have your ->dma_as set up
properly? It can be a guest thing, i.e. guest just assumes it has to do
bus addresses, while it actually does not have to, or we indeed do have
an IOMMU which polices the devices access to the guest memory, but for
some strange reason we failed to set up ->dma_as to reflect that.

@Michael: what is your opinion?

> 
> 
> In fact, I have a feeling that this is not the first time this kind of 
> situation is discussed
> around here. This reminds me of [2] and a discussion about the order virtiofs 
> features
> are negotiated versus when/how QEMU inits the devices.
> 
> 
> 
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1935019
> [2] https://lists.gnu.org/archive/html/qemu-devel/2021-02/msg05644.html
> 
> 
> Thanks,
> 
> 
> Daniel
> 
> 
> > 
> > Regards,
> > Halil
> > 
> > On Tue, 25 Jan 2022 11:21:12 +0100
> > Halil Pasic <pasic@linux.ibm.com> wrote:
> >   
> >> ping
> >>
> >> On Mon, 17 Jan 2022 13:02:38 +0100
> >> Halil Pasic <pasic@linux.ibm.com> wrote:
> >>  
> >>> The commit 04ceb61a40 ("virtio: Fail if iommu_platform is requested, but
> >>> unsupported") claims to fail the device hotplug when iommu_platform
> >>> is requested, but not supported by the (vhost) device. On the first
> >>> glance the condition for detecting that situation looks perfect, but
> >>> because a certain peculiarity of virtio_platform it ain't.
> >>>
> >>> In fact the aforementioned commit introduces a regression. It breaks
> >>> virtio-fs support for Secure Execution, and most likely also for AMD SEV
> >>> or any other confidential guest scenario that relies encrypted guest
> >>> memory.  The same also applies to any other vhost device that does not
> >>> support _F_ACCESS_PLATFORM.
> >>>
> >>> The peculiarity is that iommu_platform and _F_ACCESS_PLATFORM collates
> >>> "device can not access all of the guest RAM" and "iova != gpa, thus
> >>> device needs to translate iova".
> >>>
> >>> Confidential guest technologies currently rely on the device/hypervisor
> >>> offering _F_ACCESS_PLATFORM, so that, after the feature has been
> >>> negotiated, the guest  grants access to the portions of memory the
> >>> device needs to see. So in for confidential guests, generally,
> >>> _F_ACCESS_PLATFORM is about the restricted access to memory, but not
> >>> about the addresses used being something else than guest physical
> >>> addresses.
> >>>
> >>> This is the very reason for which commit f7ef7e6e3b ("vhost: correctly
> >>> turn on VIRTIO_F_IOMMU_PLATFORM") for, which fences _F_ACCESS_PLATFORM
> >>> form the vhost device that does not need it, because on the vhost
> >>> interface it only means "I/O address translation is needed".
> >>>
> >>> This patch takes inspiration from f7ef7e6e3b ("vhost: correctly turn on
> >>> VIRTIO_F_IOMMU_PLATFORM"), and uses the same condition for detecting the
> >>> situation when _F_ACCESS_PLATFORM is requested, but no I/O translation
> >>> by the device, and thus no device capability is needed. In this
> >>> situation claiming that the device does not support iommu_plattform=on
> >>> is counter-productive. So let us stop doing that!
> >>>
> >>> Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> >>> Reported-by: Jakob Naucke <Jakob.Naucke@ibm.com>
> >>> Fixes: 04ceb61a40 ("virtio: Fail if iommu_platform is requested, but
> >>> unsupported")
> >>> Cc: Kevin Wolf <kwolf@redhat.com>
> >>> Cc: qemu-stable@nongnu.org
> >>>
> >>> ---
> >>>
> >>> v1->v2:
> >>> * Commit message tweaks. Most notably fixed commit SHA (Michael)
> >>>
> >>> ---
> >>>   hw/virtio/virtio-bus.c | 11 ++++++-----
> >>>   1 file changed, 6 insertions(+), 5 deletions(-)
> >>>
> >>> diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
> >>> index d23db98c56..c1578f3de2 100644
> >>> --- a/hw/virtio/virtio-bus.c
> >>> +++ b/hw/virtio/virtio-bus.c
> >>> @@ -69,11 +69,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, 
> >>> Error **errp)
> >>>           return;
> >>>       }
> >>>   
> >>> -    if (has_iommu && !virtio_host_has_feature(vdev, 
> >>> VIRTIO_F_IOMMU_PLATFORM)) {
> >>> -        error_setg(errp, "iommu_platform=true is not supported by the 
> >>> device");
> >>> -        return;
> >>> -    }
> >>> -
> >>>       if (klass->device_plugged != NULL) {
> >>>           klass->device_plugged(qbus->parent, &local_err);
> >>>       }
> >>> @@ -88,6 +83,12 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, 
> >>> Error **errp)
> >>>       } else {
> >>>           vdev->dma_as = &address_space_memory;
> >>>       }
> >>> +
> >>> +    if (has_iommu && vdev->dma_as != &address_space_memory
> >>> +                  && !virtio_host_has_feature(vdev, 
> >>> VIRTIO_F_IOMMU_PLATFORM)) {
> >>> +        error_setg(errp, "iommu_platform=true is not supported by the 
> >>> device");
> >>> +        return;
> >>> +    }
> >>>   }
> >>>   
> >>>   /* Reset the virtio_bus */
> >>>
> >>> base-commit: 6621441db50d5bae7e34dbd04bf3c57a27a71b32  
> >>  
> > 
> >   
>