linux-kernel - Re: [PATCH 1/8] KVM: nSVM: Sync next

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Date:   Wed, 20 Apr 2022 18:33:07 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] KVM: nSVM: Sync next_rip field from vmcb12 to vmcb02

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