Re: [PATCH] make vfio and DAX cache work together

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Dev Audsin (dev.devaqemu@gmail.com) wrote:
> Thanks Dave for your explanation.
> Any suggestions on how to make VFIO not attempt to map into the
> unaccessible and unallocated RAM.

I'm not sure;:

static bool vfio_listener_skipped_section(MemoryRegionSection *section)
{
    return (!memory_region_is_ram(section->mr) &&
            !memory_region_is_iommu(section->mr)) ||
           section->offset_within_address_space & (1ULL << 63);
}

I'm declaring that region with memory_region_init_ram_ptr;  should I be?
it's not quite like RAM.
But then I *do* want a kvm slot for it, and I do want it to be accessed
by mapping rather htan calling IO functions; that makes me think mr->ram
has to be true.
But then do we need to add another flag to memory-regions; if we do,
what is it;
   a) We don't want an 'is_virtio_fs' - it needs to be more generic
   b) 'no_vfio' also feels wrong

Is perhaps 'not_lockable' the right thing to call it?

Dave


> Best
> Dev
> 
> On Tue, Apr 27, 2021 at 8:00 PM Dr. David Alan Gilbert
> <dgilbert@redhat.com> wrote:
> >
> > * Alex Williamson (alex.williamson@redhat.com) wrote:
> > > On Tue, 27 Apr 2021 17:29:37 +0100
> > > Dev Audsin <dev.devaqemu@gmail.com> wrote:
> > >
> > > > Hi Alex
> > > >
> > > > Based on your comments and thinking a bit, wonder if it makes sense to
> > > > allow DMA map for the DAX cache but make unexpected mappings to be not
> > > > fatal. Please let me know your thoughts.
> > >
> > > I think you're still working on the assumption that simply making the
> > > VM boot is an improvement, it's not.  If there's a risk that a possible
> > > DMA target for the device cannot be mapped, it's better that the VM
> > > fail to boot than to expose that risk.  Performance cannot compromise
> > > correctness.
> > >
> > > We do allow DMA mappings to other device memory regions to fail
> > > non-fatally with the logic that peer-to-peer DMA is often not trusted
> > > to work by drivers and therefore support would be probed before
> > > assuming that it works.  I don't think that same logic applies here.
> > >
> > > Is there something about the definition of this particular region that
> > > precludes it from being a DMA target for an assigned devices?
> >
> > It's never really the ram that's used.
> > This area is really a chunk of VMA that's mmap'd over by (chunks of)
> > normal files in the underlying exported filesystem.  The actual RAM
> > block itself is just a placeholder for the VMA, and is normally mapped
> > PROT_NONE until an actual file is mapped on top of it.
> > That cache bar is a mapping containing multiple separate file chunk
> > mappings.
> >
> > So I guess the problems for VFIO are:
> >   a) At the start it's unmapped, unaccessible, unallocated ram.
> >   b) Later it's arbitrary chunks of ondisk files.
> >
> > [on a bad day, and it's bad even without vfio, someone truncates the
> > file mapping]
> >
> > Dave
> >
> > > Otherwise if it's initially unpopulated, maybe something like the
> > > RamDiscardManager could be used to insert DMA mappings as the region
> > > becomes populated.
> > >
> > > Simply disabling mapping to boot with both features together, without
> > > analyzing how that missing mapping affects their interaction is not
> > > acceptable.  Thanks,
> > >
> > > Alex
> > >
> > > > On Mon, Apr 26, 2021 at 10:22 PM Alex Williamson
> > > > <alex.williamson@redhat.com> wrote:
> > > > >
> > > > > On Mon, 26 Apr 2021 21:50:38 +0100
> > > > > Dev Audsin <dev.devaqemu@gmail.com> wrote:
> > > > >
> > > > > > Hi Alex and David
> > > > > >
> > > > > > @Alex:
> > > > > >
> > > > > > Justification on why this region cannot be a DMA target for the device,
> > > > > >
> > > > > > virtio-fs with DAX is currently not compatible with NIC Pass through.
> > > > > > When a SR-IOV VF attaches to a qemu process, vfio will try to pin the
> > > > > > entire DAX Window but it is empty when the guest boots and will fail.
> > > > > > A method to make VFIO and DAX to work together is to make vfio skip
> > > > > > DAX cache.
> > > > > >
> > > > > > Currently DAX cache need to be set to 0, for the SR-IOV VF to be
> > > > > > attached to Kata containers. Enabling both SR-IOV VF and DAX work
> > > > > > together will potentially improve performance for workloads which are
> > > > > > I/O and network intensive.
> > > > >
> > > > > Sorry, there's no actual justification described here.  You're enabling
> > > > > a VM with both features, virtio-fs DAX and VFIO, but there's no
> > > > > evidence that they "work together" or that your use case is simply
> > > > > avoiding a scenario where the device might attempt to DMA into the area
> > > > > with this designation.  With this change, if the device were to attempt
> > > > > to DMA into this region, it would be blocked by the IOMMU, which might
> > > > > result in a data loss within the VM.  Justification of this change
> > > > > needs to prove that this region can never be a DMA target for the
> > > > > device, not simply that both features can be enabled and we hope that
> > > > > they don't interact.  Thanks,
> > > > >
> > > > > Alex
> > > > >
> > > >
> > >
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK