From dfb45db43e9f6283a79230c8ea9cb589f14791b0 Mon Sep 17 00:00:00 2001 From: Vipin Sharma Date: Wed, 21 Sep 2022 23:24:51 -0700 Subject: [PATCH] KVM: selftests: Load RAX with -EFAULT before Hyper-V hypercall Load RAX with -EFAULT prior to making a Hyper-V hypercall so that tests can't get false negatives due to the compiler coincidentally loading the "right" value into RAX, i.e. to ensure that _KVM_ and not the compiler is correctly clearing RAX on a successful hypercall. Note, initializing *hv_status (in C code) to -EFAULT is not sufficient to avoid false negatives, as the compiler can still "clobber" RAX and thus load garbage into *hv_status if the hypercall faults (or if KVM doesn't set RAX). Suggested-by: Sean Christopherson Signed-off-by: Vipin Sharma Reviewed-by: Vitaly Kuznetsov Link: https://lore.kernel.org/r/20220922062451.2927010-1-vipinsh@google.com [sean: move to separate patch, massage changelog] Signed-off-by: Sean Christopherson --- tools/testing/selftests/kvm/x86_64/hyperv_features.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_features.c b/tools/testing/selftests/kvm/x86_64/hyperv_features.c index ad01868548f9..4d55e038c2d7 100644 --- a/tools/testing/selftests/kvm/x86_64/hyperv_features.c +++ b/tools/testing/selftests/kvm/x86_64/hyperv_features.c @@ -26,7 +26,8 @@ static inline uint8_t hypercall(u64 control, vm_vaddr_t input_address, : "=a" (*hv_status), "+c" (control), "+d" (input_address), KVM_ASM_SAFE_OUTPUTS(vector) - : [output_address] "r"(output_address) + : [output_address] "r"(output_address), + "a" (-EFAULT) : "cc", "memory", "r8", KVM_ASM_SAFE_CLOBBERS); return vector; }