Re: [PATCH v3 1/1] virtio: fix the condition for iommu_platform not supported

[Halil Pasic]

On Tue, 1 Feb 2022 16:31:22 -0300
Daniel Henrique Barboza <danielhb413@gmail.com> wrote:

> On 2/1/22 15:33, Halil Pasic wrote:
> > On Tue, 1 Feb 2022 12:36:25 -0300
> > Daniel Henrique Barboza <danielhb@linux.ibm.com> wrote:
> >   
> >>> +    vdev_has_iommu = virtio_host_has_feature(vdev, 
> >>> VIRTIO_F_IOMMU_PLATFORM);
> >>>        if (klass->get_dma_as != NULL && has_iommu) {
> >>>            virtio_add_feature(&vdev->host_features, 
> >>> VIRTIO_F_IOMMU_PLATFORM);
> >>>            vdev->dma_as = klass->get_dma_as(qbus->parent);
> >>> +        if (!vdev_has_iommu && vdev->dma_as != &address_space_memory) {
> >>> +            error_setg(errp,
> >>> +                       "iommu_platform=true is not supported by the 
> >>> device");
> >>> +        }  
> >>
> >>  
> >>>        } else {
> >>>            vdev->dma_as = &address_space_memory;
> >>>        }  
> >>
> >>
> >> I struggled to understand what this 'else' clause was doing and I assumed 
> >> that it was
> >> wrong. Searching through the ML I learned that this 'else' clause is 
> >> intended to handle
> >> legacy virtio devices that doesn't support the DMA API (introduced in 
> >> 8607f5c3072caeebb)
> >> and thus shouldn't set  VIRTIO_F_IOMMU_PLATFORM.
> >>
> >>
> >> My suggestion, if a v4 is required for any other reason, is to add a small 
> >> comment in this
> >> 'else' clause explaining that this is the legacy virtio devices condition 
> >> and those devices
> >> don't set F_IOMMU_PLATFORM. This would make the code easier to read for a 
> >> virtio casual like
> >> myself.  
> > 
> > I do not agree that this is about legacy virtio. In my understanding
> > virtio-ccw simply does not need translation because CCW devices use
> > guest physical addresses as per architecture. It may be considered
> > legacy stuff form PCI perspective, but I don't think it is legacy
> > in general.  
> 
> 
> I wasn't talking about virtio-ccw. I was talking about this piece of code:
> 
> 
>      if (klass->get_dma_as != NULL && has_iommu) {
>          virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
>          vdev->dma_as = klass->get_dma_as(qbus->parent);
>      } else {
>          vdev->dma_as = &address_space_memory;
>      }
> 
> 
> I suggested something like this:
> 
> 
> 
>      if (klass->get_dma_as != NULL && has_iommu) {
>          virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
>          vdev->dma_as = klass->get_dma_as(qbus->parent);
>      } else {
>          /*
>           * We don't force VIRTIO_F_IOMMU_PLATFORM for legacy devices, i.e.
>           * devices that don't implement klass->get_dma_as, regardless of
>           * 'has_iommu' setting.
>           */
>          vdev->dma_as = &address_space_memory;
>      }
> 
> 
> At least from my reading of commits 8607f5c3072 and 2943b53f682 this seems to 
> be
> the case. I spent some time thinking that this IF/ELSE was wrong because I 
> wasn't
> aware of this history.

With virtio-ccw we take the else branch because we don't implement
->get_dma_as(). I don't consider all the virtio-ccw to be legacy.

IMHO there are two ways to think about this: 
a) The commit that introduced this needs a fix which implemets
get_dma_as() for virtio-ccw in a way that it simply returns
address_space_memory.
b) The presence of ->get_dma_as() is not indicative of "legacy".

BTW in virtospeak "legacy" has a special meaning: pre-1.0 virtio. Do you
mean that legacy. And if I read the virtio-pci code correctly
->get_dma_as is set for legacy, transitional and modern devices alike.

IMHO the important thing to figure out is what impact that
virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
in the first branch (of the if-else) has. IMHO if one examines the
commits 8607f5c307 ("virtio: convert to use DMA api") and 2943b53f68
("virtio: force VIRTIO_F_IOMMU_PLATFORM") very carefully, one will
probably reach the conclusion that the objective of the latter, is
to prevent the guest form not negotiating the IOMMU_PLATFORM feature
(clearing it as part of the feature negotiation) and trying to use
the device without that feature. In other words, virtio features are
usually optional for the guest for the sake of compatibility, but
IOMMU_PLATFORM is not: for very good reasons. Neither the commit message
nor the patch does mention legacy anywhere. 

In my opinion not forcing the guest to negotiate IOMMU_PLATFORM when
->get_dma_as() is not set is at least unfortunate. Please observe, that
virtio-pci is not affected by this omission because for virtio-pci
devices ->get_dma_as != NULL always holds. And what is the deal for
devices that don't implement get_dma_as() (and don't need address
translation)? If iommu_platform=on is justified (no user error) then
the device does not have access to the entire guest memory. Which
means it more than likely needs cooperation form the guest (driver).
So detecting that the guest does not support IOMMU_PLATFORM and failing
gracefully via virtio_validate_features() instead of carrying on
in good faith and failing in ugly ways when the host attempts to access
guest memory to which it does not have access to. If we assume user
error, that is the host can access at least all the memory it needs
to access to make that device work, then it is probably still a
good idea to fail the device and thus help the user correct his
error.

IMHO the best course of action is
diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index 34f5a0a664..1d0eb16d1c 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -80,7 +80,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error 
**errp)
 
     vdev_has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
     if (klass->get_dma_as != NULL && has_iommu) {
-        virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
         vdev->dma_as = klass->get_dma_as(qbus->parent);
         if (!vdev_has_iommu && vdev->dma_as != &address_space_memory) {
             error_setg(errp,
@@ -89,6 +88,7 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error 
**errp)
     } else {
         vdev->dma_as = &address_space_memory;
     }
+    virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
 }

which would be a separate patch, as this is a separate issue. Jason,
Michael, Connie, what do you think?

Regards,
Halil