Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[Sean Christopherson]

 On Wed, Apr 08, 2020 at 10:23:58AM +0200, Paolo Bonzini wrote:
> Page-not-present async page faults are almost a perfect match for the
> hardware use of #VE (and it might even be possible to let the processor
> deliver the exceptions).

My "async" page fault knowledge is limited, but if the desired behavior is
to reflect a fault into the guest for select EPT Violations, then yes,
enabling EPT Violation #VEs in hardware is doable.  The big gotcha is that
KVM needs to set the suppress #VE bit for all EPTEs when allocating a new
MMU page, otherwise not-present faults on zero-initialized EPTEs will get
reflected.

Attached a patch that does the prep work in the MMU.  The VMX usage would be:

	kvm_mmu_set_spte_init_value(VMX_EPT_SUPPRESS_VE_BIT);

when EPT Violation #VEs are enabled.  It's 64-bit only as it uses stosq to
initialize EPTEs.  32-bit could also be supported by doing memcpy() from
a static page.