Re: [PATCH 01/16] KVM: x86/mmu: Exit to userspace with -EFAULT if private fault hits emulation

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 12, 2024, Binbin Wu wrote:
> 
> On 3/7/2024 8:52 PM, Gupta, Pankaj wrote:
> > > Exit to userspace with -EFAULT / KVM_EXIT_MEMORY_FAULT if a private fault
> > > triggers emulation of any kind, as KVM doesn't currently support
> > > emulating access to guest private memory.  Practically speaking, private
> > > faults and emulation are already mutually exclusive, but there are edge
> > > cases upon edge cases where KVM can return RET_PF_EMULATE, and adding one
> > > last check
> > 
> > edge cases upon edge cases?
> > 
> > Just curious about the details of the edge cases scenarios?
> 
> Same question out of curiosity.

Accesses that hit the APIC-access page and gfns that are write-tracked, are the
two most likely candidates.  Even plain old emulated MMIO falls into this bucket,
e.g. if KVM botched things and generated a RSVD fault on a private mapping.  I'll
reword that line to

  faults and emulation are already mutually exclusive, but there are many
  flows that can result in KVM returning RET_PF_EMULATE, and adding one last
  check to harden against weird, unexpected combinations and/or KVM bugs is
  inexpensive.

to make it sound less dramatic/hand-wavy.