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SubjectRe: [PATCH 6/8] kvm: svm: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to guest
On Tue, Jan 09, 2018 at 01:03:08PM +0100, 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
> @@ -183,6 +183,8 @@ struct vcpu_svm {
> u64 gs_base;
> } host;
>
> + u64 spec_ctrl;
> +
> u32 *msrpm;
>
> ulong nmi_iret_rip;
> @@ -248,6 +250,8 @@ struct amd_svm_iommu_ir {
> { .index = MSR_CSTAR, .always = true },
> { .index = MSR_SYSCALL_MASK, .always = true },
> #endif
> + { .index = MSR_IA32_SPEC_CTRL, .always = true },
> + { .index = MSR_IA32_PRED_CMD, .always = true },
> { .index = MSR_IA32_LASTBRANCHFROMIP, .always = false },
> { .index = MSR_IA32_LASTBRANCHTOIP, .always = false },
> { .index = MSR_IA32_LASTINTFROMIP, .always = false },
> @@ -283,6 +287,8 @@ struct amd_svm_iommu_ir {
> /* enable/disable Virtual GIF */
> static int vgif = true;
> module_param(vgif, int, 0444);
> +
> +static bool __read_mostly have_spec_ctrl;
>
> static void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0);
> static void svm_flush_tlb(struct kvm_vcpu *vcpu, bool invalidate_gpa);
> @@ -1135,6 +1141,17 @@ static __init int svm_hardware_setup(void)
> pr_info("Virtual GIF supported\n");
> }
>
> + /*
> + * FIXME: this is only needed until SPEC_CTRL is supported
> + * by upstream Linux in cpufeatures, then it can be replaced
> + * with static_cpu_has.
> + */
> + have_spec_ctrl = cpu_has_spec_ctrl();
> + if (have_spec_ctrl)
> + pr_info("kvm: SPEC_CTRL available\n");
> + else
> + pr_info("kvm: SPEC_CTRL not available\n");

Perhaps just

pr_info("kvm: SPEC_CTRL %s available\n", have_spec_ctrl ? "" : "not");

> +
> return 0;
>
> err:
> @@ -3599,6 +3616,9 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> case MSR_VM_CR:
> msr_info->data = svm->nested.vm_cr_msr;
> break;
> + case MSR_IA32_SPEC_CTRL:
> + msr_info->data = svm->spec_ctrl;
> + break;
> case MSR_IA32_UCODE_REV:
> msr_info->data = 0x01000065;
> break;
> @@ -3754,6 +3774,9 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
> case MSR_VM_IGNNE:
> vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n", ecx, data);
> break;
> + case MSR_IA32_SPEC_CTRL:
> + svm->spec_ctrl = data;
> + break;
> case MSR_IA32_APICBASE:
> if (kvm_vcpu_apicv_active(vcpu))
> avic_update_vapic_bar(to_svm(vcpu), data);
> @@ -4942,6 +4965,13 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>
> local_irq_enable();
>
> + /*
> + * MSR_IA32_SPEC_CTRL is restored after the last indirect branch
> + * before vmentry.
> + */
> + if (have_spec_ctrl && svm->spec_ctrl != 0)
> + wrmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
> +
> asm volatile (
> "push %%" _ASM_BP "; \n\t"
> "mov %c[rbx](%[svm]), %%" _ASM_BX " \n\t"
> @@ -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)

Perhaps just :

if (svm->spec_ctrl) ?

And above too?
> + 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");

Don't you want this to be part of the if () .. else part?

Meaning:

if (have_spec_ctrl && svm->spec_ctrl)
wrmsrl(MSR_IA32_SPEC_CTRL, 0);
else
asm("lfence");

But .. I am missing something - AMD don't expose 0x48. They expose only 0x49.

That is only the IPBP is needed on AMD? (I haven't actually seen any official
docs from AMD).

> +
> #ifdef CONFIG_X86_64
> wrmsrl(MSR_GS_BASE, svm->host.gs_base);
> #else
> --
> 1.8.3.1
>
>

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