Re: [PATCH V2] vhost: correctly turn on VIRTIO_F_IOMMU_PLATFORM

[Michael S. Tsirkin]

On Fri, Mar 13, 2020 at 03:27:59PM -0500, Brijesh Singh wrote:
> 
> On 3/13/20 7:44 AM, Halil Pasic wrote:
> > [..]
> >>> CCing Tom. @Tom does vhost-vsock work for you with SEV and current qemu?
> >>>
> >>> Also, one can specify iommu_platform=on on a device that ain't a part of
> >>> a secure-capable VM, just for the fun of it. And that breaks
> >>> vhost-vsock. Or is setting iommu_platform=on only valid if
> >>> qemu-system-s390x is protected virtualization capable?
> >>>
> >>> BTW, I don't have a strong opinion on the fixes tag. We currently do not
> >>> recommend setting iommu_platform, and thus I don't think we care too
> >>> much about past qemus having problems with it.
> >>>
> >>> Regards,
> >>> Halil
> >>
> >> Let's just say if we do have a Fixes: tag we want to set it correctly to
> >> the commit that needs this fix.
> >>
> > I finally did some digging regarding the performance degradation. For
> > s390x the performance degradation on vhost-net was introduced by commit
> > 076a93d797 ("exec: simplify address_space_get_iotlb_entry"). Before
> > IOMMUTLBEntry.addr_mask used to be based on plen, which in turn was
> > calculated as the rest of the memory regions size (from address), and
> > covered most of the guest address space. That is we didn't have a whole
> > lot of IOTLB API overhead.
> >
> > With commit 076a93d797 I see IOMMUTLBEntry.addr_mask == 0xfff which comes
> > as ~TARGET_PAGE_MASK from flatview_do_translate(). To have things working
> > properly I applied 75e5b70e6, b021d1c044, and d542800d1e on the level of
> > 076a93d797 and 076a93d797~1.
> >
> > Regarding vhost-vsock. It does not work with iommu_platform=on since the
> > very beginning (i.e. 8607f5c307 ("virtio: convert to use DMA api")). Not
> > sure if that is a good or a bad thing. (If the vhost driver in the kernel
> > would actually have to do the IOTLB translation, then failing in case
> > where it does not support it seems sane. The problem is that
> > ACCESS_PLATFORM is used for more than one thing (needs translation, and
> > restricted memory access).)
> >
> > I don't think I've heard back from AMD whether vsock works with SEV or
> > not... I don't have access to HW to test it myself.
> 
> 
> I just tried vhost-vsock on AMD SEV machine and it does not work. I am
> using FC31 (qemu 4.1.1.1.fc31).

Neither does vhost scsi - no ACCESS_PLATFORM support. But with Jason's
patch I think both should work. Pls give it a try.

> 
> > We (s390) don't require this being backported to the stable qemus,
> > because for us iommu_platform=on becomes relevant with protected
> > virtualization, and those qemu versions don't support it.
> >
> > Cheers,
> > Halil
> >