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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 14 Dec 2020 12:08:42 -0800
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Subject: Re: [PATCH v2] KVM: SVM: use vmsave/vmload for saving/restoring
 additional host state
To:     Sean Christopherson <seanjc@google.com>
Cc:     Michael Roth <michael.roth@amd.com>,
        kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        X86 ML <x86@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Andy Lutomirski <luto@kernel.org>
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On Mon, Dec 14, 2020 at 11:38 AM Sean Christopherson <seanjc@google.com> wrote:
>
> +Andy, who provided a lot of feedback on v1.
>
> >
> >  static unsigned long svm_get_rflags(struct kvm_vcpu *vcpu)
> > @@ -3507,14 +3503,8 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> >
> >       __svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
>
> Tying in with avoiding svm->host_save_area, what about passing in the PA of the
> save area and doing the vmload in __svm_vcpu_run()?  One less instance of inline
> assembly to stare at...

One potential side benefit is that we wouldn't execute any C code with
the wrong MSR_GS_BASE, which avoids any concerns about
instrumentation, stack protector, or some *SAN feature exploding due
to a percpu memory not working.

--Andy