Re: [PATCH v3 6/8] admin: Add theory of operation for write recording commands

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 16, 2023 at 12:24:27PM +0800, Jason Wang wrote:
> On Thu, Nov 16, 2023 at 1:37âAM Parav Pandit <parav@nvidia.com> wrote:
> >
> >
> > > From: Jason Wang <jasowang@redhat.com>
> > > Sent: Monday, November 13, 2023 9:11 AM
> > >
> > > On Fri, Nov 10, 2023 at 2:46âPM Parav Pandit <parav@nvidia.com> wrote:
> > > >
> > > > Hi Michael,
> > > >
> > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > Sent: Thursday, November 9, 2023 1:29 PM
> > > >
> > > > [..]
> > > > > > Besides the issue of performance, it's also racy, assuming we are
> > > > > > logging
> > > > > IOVA.
> > > > > >
> > > > > > 0) device log IOVA
> > > > > > 1) hypervisor fetches IOVA from log buffer
> > > > > > 2) guest map IOVA to a new GPA
> > > > > > 3) hypervisor traverse guest table to get IOVA to new GPA
> > > > > >
> > > > > > Then we lost the old GPA.
> > > > >
> > > > > Interesting and a good point. And by the way e.g. vhost has the same
> > > > > issue.  You need to flush dirty tracking info when changing the
> > > > > mappings somehow.  Parav what's the plan for this? Should be addressed in
> > > the spec too.
> > > > >
> > > > As you listed the flush is needed for vhost or device-based DPT.
> > >
> > > What does DPT mean? Device Page Table? Let's not invent terminology which is
> > > not known by others please.
> > >
> > Sorry for using the acronym. I meant dirty page tracking.
> >
> > > We have discussed it many times. You can't just depend on ATS or reinventing
> > > wheels in virtio.
> > The dependency is on the iommu which would have the mapping of GIOVA to GPA like any sw implementation.
> > No dependency on ATS.
> >
> > >
> > > What's more, please try not to give me the impression that the proposal is
> > > optimized for a specific vendor (like device IOMMU stuffs).
> > >
> > You should stop calling this specific vendor thing.
> 
> Well, as you have explained, the confusion came from "DPT" ...
> 
> > One can equally say that suspend bit proposal is for the sw_vendor device who is forcing virtio hw device to only implement ioqueues + PASID + non_unified interface for PF, VF, SIOVs + non_TDISP based devices.
> >
> > > > The necessary plumbing is already covered for this in the query (read and
> > > clear) command of this v3 proposal.
> > >
> > > The issue is logging via IOVA ... I don't see how "read and clear" can help.
> > >
> > Read and clear helps that ensures that all the dirty pages are reported, hence there is no mapping/unmapping race.
> 
> Reported as IOVA ...
> 
> > As everything is reported.
> >
> > > > It is listed in Device Write Records Read Command.
> > >
> > > Please explain how your proposal can solve the above race.
> > >
> > In below manner.
> > 1. guest has GIOVA to GPA_1 mapping
> > 2. RX packets occurred to GIOVA
> > 3. device reported dirty page log for GIOVA (hypervisor is yet to read)
> > 4. guest requested mapping change from GIOVA to GPA_2
> > 4.1 During this IOTLB is invalidated and dirty page report is queried ensuring, it can change the mapping
> 
> It requires
> 
> 1) hypervisor traps IOTLB invalidation, which doesn't work when
> nesting could be offloaded (IOMMUFD has started the work to support
> nesting)
> 2) query the device about the dirty page on each IOTLB invalidation which:
> 2.1) A huge round trip: guest IOTLB invalidation -> trapped by
> hypervisor -> start the query from the device -> device return ->
> hypervisor reports IOTLB invalidation is done -> let guest run. Have
> you benchmarked the RTT in this case? There are just too many places
> that cause the delay in the middle.

To be fair invalidations are already expensive e.g. with vhost iotlb
it requires a slow system call.
This will make them *even more* expensive.

Problem for some but not all workloads.  Again I agree motivation,
tradeoffs and comparison with both dirty tracking by iommu and shadow vq
approaches really should be included.


> 2.2) Guest triggerable behaviour, malicious guest can simply do
> endless IOTLB invalidation to DOS the e.g admin virtqueue

I'm not sure how much to worry about it - just don't allow more
than one in flight per VM.



> >
> > > >
> > > > When the page write record is fully read, it is flushed.
> > > > How/when to use, I think its hypervisor specific, so we probably better off not
> > > documenting those details.
> > >
> > > Well, as the author of this proposal, at least you need to know how a hypervisor
> > > can work with your proposal, no?
> > >
> > Likely yes, but it is not the scope of the spec to list those paths etc.
> 
> Fine, but as a reviewer I need to know if it can work with a hypervisor well.
> 
> >
> > > > May be such read is needed in some other path too depending on how
> > > hypervisor implemented.
> > >
> > > What do you mean by "May be ... some other path" here? You're inventing a
> > > mechanism that you don't know how a hypervisor can use?
> >
> > No. I meant hypervisor may have more operations that map/unmap/flush where it may need to implement it.
> > Some one may call it set_map(), some may say dma_map()...
> 
> Ok.
> 
> Thanks