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SubjectRe: [PATCH 6/8] kvm: svm: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to guest
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On 1/9/2018 6:03 AM, Paolo Bonzini wrote:
> Direct access to MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD is important
> for performance. Allow load/store of MSR_IA32_SPEC_CTRL, restore guest
> IBRS on VM entry and set it to 0 on VM exit (because Linux does not use
> it yet).
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/svm.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 42 insertions(+)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 31ace8d7774a..934a21e02e03 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c

...

> @@ -5015,6 +5045,18 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
> #endif
> );
>
> + if (have_spec_ctrl) {
> + rdmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
> + if (svm->spec_ctrl != 0)
> + wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> + }
> + /*
> + * Speculative execution past the above wrmsrl might encounter
> + * an indirect branch and use guest-controlled contents of the
> + * indirect branch predictor; block it.
> + */
> + asm("lfence");

This will end up needing to be an alternative macro based on the
LFENCE_RDTSC or MFENCE_RDTSC features [1]. You'll probably just want to
use the speculation barrier macro that ends up being defined to control
the speculation here.

Thanks,
Tom

[1] https://marc.info/?l=linux-kernel&m=151545930207815&w=2

> +
> #ifdef CONFIG_X86_64
> wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> #else
>

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