Re: [RFC PATCH] KVM: Introduce KVM VIRTIO device

[Sean Christopherson <seanjc@google.com>]

+Yiwei

On Fri, Dec 15, 2023, Kevin Tian wrote:
> > From: Zhao, Yan Y <yan.y.zhao@intel.com>
> > Sent: Thursday, December 14, 2023 6:35 PM
> > 
> > - For host non-MMIO pages,
> >   * virtio guest frontend and host backend driver should be synced to use
> >     the same memory type to map a buffer. Otherwise, there will be
> >     potential problem for incorrect memory data. But this will only impact
> >     the buggy guest alone.
> >   * for live migration,
> >     as QEMU will read all guest memory during live migration, page aliasing
> >     could happen.
> >     Current thinking is to disable live migration if a virtio device has
> >     indicated its noncoherent state.
> >     As a follow-up, we can discuss other solutions. e.g.
> >     (a) switching back to coherent path before starting live migration.
> 
> both guest/host switching to coherent or host-only?
> 
> host-only certainly is problematic if guest is still using non-coherent.
> 
> on the other hand I'm not sure whether the host/guest gfx stack is
> capable of switching between coherent and non-coherent path in-fly
> when the buffer is right being rendered.
> 
> >     (b) read/write of guest memory with clflush during live migration.
> 
> write is irrelevant as it's only done in the resume path where the
> guest is not running.
> 
> > 
> > Implementation Consideration
> > ===
> > There is a previous series [1] from google to serve the same purpose to
> > let KVM be aware of virtio GPU's noncoherent DMA status. That series
> > requires a new memslot flag, and special memslots in user space.
> > 
> > We don't choose to use memslot flag to request honoring guest memory
> > type.
> 
> memslot flag has the potential to restrict the impact e.g. when using
> clflush-before-read in migration?

Yep, exactly.  E.g. if KVM needs to ensure coherency when freeing memory back to
the host kernel, then the memslot flag will allow for a much more targeted
operation.

> Of course the implication is to honor guest type only for the selected slot
> in KVM instead of applying to the entire guest memory as in previous series
> (which selects this way because vmx_get_mt_mask() is in perf-critical path
> hence not good to check memslot flag?)

Checking a memslot flag won't impact performance.  KVM already has the memslot
when creating SPTEs, e.g. the sole caller of vmx_get_mt_mask(), make_spte(), has
access to the memslot.

That isn't coincidental, KVM _must_ have the memslot to construct the SPTE, e.g.
to retrieve the associated PFN, update write-tracking for shadow pages, etc.

I added Yiwei, who I think is planning on posting another RFC for the memslot
idea (I actually completely forgot that the memslot idea had been thought of and
posted a few years back).

> > Instead we hope to make the honoring request to be explicit (not tied to a
> > memslot flag). This is because once guest memory type is honored, not only
> > memory used by guest virtio device, but all guest memory is facing page
> > aliasing issue potentially. KVM needs a generic solution to take care of
> > page aliasing issue rather than counting on memory type of a special
> > memslot being aligned in host and guest.
> > (we can discuss what a generic solution to handle page aliasing issue will
> > look like in later follow-up series).
> > 
> > On the other hand, we choose to introduce a KVM virtio device rather than
> > just provide an ioctl to wrap kvm_arch_[un]register_noncoherent_dma()
> > directly, which is based on considerations that
> 
> I wonder it's over-engineered for the purpose.
> 
> why not just introducing a KVM_CAP and allowing the VMM to enable?
> KVM doesn't need to know the exact source of requiring it...

Agreed.  If we end up needing to grant the whole VM access for some reason, just
give userspace a direct toggle.