Messages in this thread | | | Date | Wed, 20 Apr 2022 18:33:07 +0200 | From | Paolo Bonzini <> | Subject | Re: [PATCH 1/8] KVM: nSVM: Sync next_rip field from vmcb12 to vmcb02 |
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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
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