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

[Halil Pasic]

On Fri, 13 Mar 2020 12:31:22 -0400
Peter Xu <address@hidden> wrote:

> On Fri, Mar 13, 2020 at 11:29:59AM -0400, Michael S. Tsirkin wrote:
> > On Fri, Mar 13, 2020 at 01:44:46PM +0100, 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.
> > 
> > Peter, what's your take on this one?
> 
> Commit 076a93d797 was one of the patchset where we want to provide
> sensible IOTLB entries and also that should start to work with huge
> pages.  Frankly speaking after a few years I forgot the original
> motivation of that whole thing, but IIRC there's a patch that was
> trying to speedup especially for vhost but I noticed it's not merged:
> 
> https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg00574.html
> 

From the looks of it, I don't think we would have seen that big
performance degradation had this patch been included. I can give
it a spin if you like. Shall I?

> Regarding to the current patch, I'm not sure I understand it
> correctly, but is that performance issue only happens when (1) there's
> no intel-iommu device, and (2) there is iommu_platform=on specified
> for the vhost backend?
> 

I can confirm, that your description covers my scenario. I didn't
investigate what happens when we have an intel-iommu, because s390 does
not do intel-iommu. I can also confirm that no performance degradation
is observed when the virtio-net has iommu_platform=off. The property
iommu_platform is a virtio device (and not a backend) level property.
 

> If so, I'd confess I am not too surprised if this fails the boot with
> vhost-vsock because after all we speicified iommu_platform=on
> explicitly in the cmdline, so if we want it to work we can simply
> remove that iommu_platform=on when vhost-vsock doesn't support it
> yet...  I thougth iommu_platform=on was added for that case - when we
> want to force IOMMU to be enabled from host side, and it should always
> be used with a vIOMMU device.
> 

The problem is that the virtio feature bit F_ACCESS_PLATFORM, which is
directly controlled by the iommu_platform proprerty stands for two things
1) need to do IOVA translation
2) the access of the device to the guests RAM is restricted.

There are cases where 2) does apply and 1) does not. We need to specify
iommu_platform=on to make the virtio implementation in the guest use
the dma api, because we need to grant access to memory as required. But
we don't need translation and we don't have a vIOMMU.

Regards,
Halil


> However I also agree that from performance POV this patch helps for
> this quite special case.
> 
> Thanks,
>