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

[Michael S. Tsirkin]

On Fri, Mar 13, 2020 at 12:31:22PM -0400, Peter Xu 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.

So the issue bundamentally is that it
never produces entries larger than page size.

Wasteful even just with huge pages, all the more
so which passthrough which could have giga-byte
entries.

Want to try fixing that?


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