Re: [PATCH v5 4/8] KVM: nSVM: Propagate hardware DecodeAssist bytes to VMCB12

[Tina Zhang]

On 9/5/2026 7:58 AM, Jim Mattson wrote:
> On Mon, Aug 24, 2026 at 5:40 AM Tina Zhang <zhang_wei@xxxxxxxxxxxxxx> wrote:
> > +static void nested_svm_copy_insn_bytes(struct vmcb *to,
> > +                                      const struct vmcb *from)
> > +{
> > +       u8 insn_len = from->control.insn_len;
> > +
> > +       nested_svm_clear_insn_bytes(to);
>
> It's not obvious to me that this clearing is necessary (except in the
> case of the early return below). The APM does not say what happens to
> the remaining bytes if there is a short read.

You're right.  I had interpreted the APM as requiring the entire Guest 
Instruction Bytes field to be cleared when rebuilding the state, but 
that was too broad.  For a short read, the remaining bytes are not 
defined and do not need to be cleared.  Thanks for correcting my 
understanding.


> > +static void nested_svm_update_vmcb12_insn_bytes(struct kvm_vcpu *vcpu,
> > +                                               struct vmcb *vmcb12,
> > +                                               const struct vmcb *vmcb02)
> > +{
> > +       struct vcpu_svm *svm = to_svm(vcpu);
> > +
> > +       nested_svm_clear_insn_bytes(vmcb12);
>
> Clearing here is premature. If L1 does not have
> X86_FEATURE_DECODEASSISTS, the "Guest Instruction Bytes" fields of
> vmcs12 should not be touched.

Agreed.

> Moreover, as I pointed out earlier, if L1 has
> X86_FEATURE_DECODEASSISTS, and the VM-exit doesn't have instruction
> bytes, you onlyhave to clear bits 7:0.

Yes. I'll update it in the next version.


Thanks,
Tina