Re: [PATCH v3 6/6] KVM: x86: allow defining return-0 static calls

Maxim Levitsky
2022-03-20T13:59:06Z

On Fri, 2022-03-18 at 17:29 +0100, Paolo Bonzini wrote:
> On 3/17/22 18:43, Maxim Levitsky wrote:
> > diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> > index 20f64e07e359..3388072b2e3b 100644
> > --- a/arch/x86/include/asm/kvm-x86-ops.h
> > +++ b/arch/x86/include/asm/kvm-x86-ops.h
> > @@ -88,7 +88,7 @@ KVM_X86_OP(deliver_interrupt)
> > KVM_X86_OP_OPTIONAL(sync_pir_to_irr)
> > KVM_X86_OP_OPTIONAL_RET0(set_tss_addr)
> > KVM_X86_OP_OPTIONAL_RET0(set_identity_map_addr)
> > -KVM_X86_OP_OPTIONAL_RET0(get_mt_mask)
> > +KVM_X86_OP(get_mt_mask)
> > KVM_X86_OP(load_mmu_pgd)
> > KVM_X86_OP(has_wbinvd_exit)
> > KVM_X86_OP(get_l2_tsc_offset)
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index a09b4f1a18f6..0c09292b0611 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -4057,6 +4057,11 @@ static bool svm_has_emulated_msr(struct kvm *kvm, u32 index)
> > return true;
> > }
> >
> > +static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
> > +{
> > + return 0;
> > +}
> > +
> > static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > {
> > struct vcpu_svm *svm = to_svm(vcpu);
> > @@ -4718,6 +4723,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
> > .check_apicv_inhibit_reasons = avic_check_apicv_inhibit_reasons,
> > .apicv_post_state_restore = avic_apicv_post_state_restore,
> >
> > + .get_mt_mask = svm_get_mt_mask,
> > .get_exit_info = svm_get_exit_info,
> >
> > .vcpu_after_set_cpuid = svm_vcpu_after_set_cpuid,
>
> Thanks, I'll send it as a complete patch. Please reply there with your
> Signed-off-by.

Honestly, I haven't meant to include this as a fix, but only as a proof of the issue,
but I don't have anything against using this until the underlying issue is fixed.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky

>
> Related to this, I don't see anything in arch/x86/kernel/static_call.c
> that limits this code to x86-64:
>
> if (func == &__static_call_return0) {
> emulate = code;
> code = &xor5rax;
> }
>
>
> On 32-bit, it will be patched as "dec ax; xor eax, eax" or something
> like that. Fortunately it doesn't corrupt any callee-save register but
> it is not just a bit funky, it's also not a single instruction.
>
> Paolo
>