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

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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.

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




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