Re: [PATCH 0/2] KVM: nSVM: Expose DecodeAssists to L1

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 07, 2026, Jim Mattson wrote:
> On Tue, Jul 7, 2026 at 7:41 AM Sean Christopherson <seanjc@google.com> wrote:
> >
> >         insn = ctxt->fetch.data;
> >         if (ctxt->eip == kvm_rip_read(vcpu))
> >                 insn_len = ctxt->fetch.end - ctxt->fetch.ptr;
> >
> >         for (len = X86_MAX_INSTRUCTION_LENGTH - insn_len; len >= 0; len--) {
> >                 if (!kvm_fetch_guest_virt(vcpu, kvm_rip_read(vcpu) + insn_len,
> >                                           &insn[insn_len], len, NULL))
> >                         break;
> >         }
> 
> This split doesn't bother you?

What split?  Do you mean reading some bytes from the emulator, and some at the
time of #VMEXIT?  If so: no.  IMO, the most important thing in practice is to
capture the actual bytes used to decode the instruction that led to the #NPF.

Exactly how and when the "CPU" reads the bytes is a micro-architectural detail,
so I don't have any qualms from an architectural-correctness perspective.