Re: [PATCH 1/8] KVM: nSVM: Sync next_rip field from vmcb12 to vmcb02

Paolo Bonzini
2022-04-20T16:33:22Z

On 4/20/22 18:15, Sean Christopherson wrote:
>>> Let's just require X86_FEATURE_NRIPS, either in general or just to
>>> enable nested virtualiazation
>> 👍
> Hmm, so requiring NRIPS for nested doesn't actually buy us anything. KVM still
> has to deal with userspace hiding NRIPS from L1, so unless I'm overlooking something,
> the only change would be:
>
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index bdf8375a718b..7bed4e05aaea 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -686,7 +686,7 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
> */
> if (svm->nrips_enabled)
> vmcb02->control.next_rip = svm->nested.ctl.next_rip;
> - else if (boot_cpu_has(X86_FEATURE_NRIPS))
> + else
> vmcb02->control.next_rip = vmcb12_rip;
>
> if (is_evtinj_soft(vmcb02->control.event_inj)) {
>
> And sadly, because SVM doesn't provide the instruction length if an exit occurs
> while vectoring a software interrupt/exception, making NRIPS mandatory doesn't buy
> us much either.
>
> I believe the below diff is the total savings (plus the above nested thing) against
> this series if NRIPS is mandatory (ignoring the setup code, which is a wash). It
> does eliminate the rewind in svm_complete_soft_interrupt() and the funky logic in
> svm_update_soft_interrupt_rip(), but that's it AFAICT. The most obnoxious code of
> having to unwind EMULTYPE_SKIP when retrieving the next RIP for software int/except
> injection doesn't go away:-(
>
> I'm not totally opposed to requiring NRIPS, but I'm not in favor of it either.

Yeah, you're right. However:

* the rewind might already be worth it;

* if we require NRIPS for nested, we can also assume that the SVM save
state data has a valid next_rip; even if !svm->nrips_enabled. There's
the pesky issue of restoring from an old system that did not have NRIPS,
but let's assume for now that NRIPS was set on the source as well.

Paolo