Re: [RFC 29/33] KVM: VMX: Save instruction length on EPT violation

[Sean Christopherson]

On Wed, Nov 08, 2023, Nicolas Saenz Julienne wrote:
> Save the length of the instruction that triggered an EPT violation in
> struct kvm_vcpu_arch. This will be used to populate Hyper-V VSM memory
> intercept messages.

This is silly and unnecessarily obfuscates *why* (as my response regarding SVM
shows), i.e. that this is "needed" becuase the value is consumed by a *different*
vCPU, not because of performance concerns.

It's also broken, AFAICT nothing prevents the intercepted vCPU from hitting a
different EPT violation before the target vCPU consumes exit_instruction_len.

Holy cow.  All of deliver_gpa_intercept() is wildly unsafe.  Aside from race
conditions, which in and of themselves are a non-starter, nothing guarantees that
the intercepted vCPU actually cached all of the information that is held in its VMCS.

The sane way to do this is to snapshot *all* information on the intercepted vCPU,
and then hand that off as a payload to the target vCPU.  That is, assuming the
cross-vCPU stuff is actually necessary.  At a glance, I don't see anything that
explains *why*.