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

[Peter Xu]

On Mon, Mar 16, 2020 at 01:19:54PM -0400, Michael S. Tsirkin wrote:
> 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?

Yes we can fix that, but I'm still not sure whether changing the
interface of address_space_get_iotlb_entry() to cover adhoc regions is
a good idea, because I think it's still a memory core API and imho it
would still be good to have IOTLBs returned to be what the hardware
will be using (always page aligned IOTLBs).  Also it would still be
not ideal because vhost backend will still need to send the MISSING
messages and block for each of the continuous guest memory ranges
registered, so there will still be misterious delay.  Not to say
logically all the caches can be invalidated too so in that sense I
think it's as hacky as the vhost speedup patch mentioned below..

Ideally I think vhost should be able to know when PT is enabled or
disabled for the device, so the vhost backend (kernel or userspace)
should be able to directly use GPA for DMA.  That might need some new
vhost interface.

For the s390's specific issue, I would think Jason's patch an simple
and ideal solution already.

Thanks,

> 
> 
> >  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
> 

-- 
Peter Xu