From: Sean Christopherson <seanjc@google.com>
To: Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
f734222792@gmail.com, Vitaly Kuznetsov <vkuznets@redhat.com>,
Sashiko Bot <sashiko-bot@kernel.org>,
Yosry Ahmed <yosry@kernel.org>
Subject: [PATCH v4 03/17] KVM: selftests: Don't clobber RFLAGS in happy path of __KVM_ASM_SAFE()
Date: Thu, 3 Sep 2026 17:24:09 -0700 [thread overview]
Message-ID: <20260904002423.3154436-4-seanjc@google.com> (raw)
In-Reply-To: <20260904002423.3154436-1-seanjc@google.com>
Use a straight MOV instead of an XOR to zero R9 in the non-faulting path of
__KVM_ASM_SAFE() so as not to clobber RFLAGS. XOR generates smaller code,
but optimizing performance by minimizing the code footprint is just about
the last thing that KVM selftests care about.
The bug is relatively benign in the current code base, e.g. it marginally
degrades the coverage in the fastops test, but otherwise does no harm. But
the bug will become actively problematic if/when the "safe assembly" macros
are used for instructions that communicate results via RFLAGS, e.g. for VMX
instructions, which signal failure via RFLAGS.CF and RFLAGS.ZF.
Opportunistically use R9D instead of the full R9, as bits 63:32 of GPRs are
zeroed on writes to bit 31:0.
Reported-by: Sashiko Bot <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260818165258.2613603-5-vkuznets@redhat.com
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
tools/testing/selftests/kvm/include/x86/processor.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tools/testing/selftests/kvm/include/x86/processor.h b/tools/testing/selftests/kvm/include/x86/processor.h
index 6e6f70035508..282b8aaff799 100644
--- a/tools/testing/selftests/kvm/include/x86/processor.h
+++ b/tools/testing/selftests/kvm/include/x86/processor.h
@@ -1318,7 +1318,7 @@ gva_t vm_alloc_stack(struct kvm_vm *vm, int nr_pages);
"lea 1f(%%rip), %%r10\n\t" \
"lea 2f(%%rip), %%r11\n\t" \
fep "1: " insn "\n\t" \
- "xor %%r9, %%r9\n\t" \
+ "mov $0, %%r9d\n\t" \
"2:\n\t" \
"mov %%r9b, %[vector]\n\t" \
"mov %%r10, %[error_code]\n\t"
--
2.55.0.979.g7e5102b832-goog
next prev parent reply other threads:[~2026-09-04 0:24 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-04 0:24 [PATCH v4 00/17] KVM: nVMX: Adjust VMPTRLD/VMPTRST behavior with active eVMCS Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 01/17] KVM: nVMX: Make VMPTRLD result in #UD when eVMCS is used Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 02/17] KVM: nVMX: Make VMPTRST return eVMCS GPA when it " Sean Christopherson
2026-09-04 0:24 ` Sean Christopherson [this message]
2026-09-04 0:24 ` [PATCH v4 04/17] KVM: selftests: Adapt to the updated VMPTRST behavior when eVMCS " Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 05/17] KVM: selftests: Check VMPTRLD with active eVMCS Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 06/17] KVM: selftests: Assert success in vmptrst(), kill off vmptrstz() Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 07/17] KVM: selftests: Always assert that vmxon() and prepare_for_vmx_operation() succeed Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 08/17] KVM: selftests: Always assert that vmclear() succeeds Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 09/17] KVM: selftests: Always assert that vmptrld() succeeds Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 10/17] KVM: selftests: Drop useless return code from load_vmcs() Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 11/17] KVM: selftests: Add macros to handle simple VMX instructions Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 12/17] KVM: selftests: Drop dead return code from evmcs_vmptrld() and load_evmcs() Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 13/17] KVM: selftests: Dedup assembly code for VMLAUNCH and VMRESUME Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 14/17] KVM: selftests: Assert success in vmlaunch() and vmresume() Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 15/17] KVM: selftests: Add __vmwrite() and use it when initializing optional fields Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 16/17] KVM: selftests: Always assert that vmwrite() succeeds Sean Christopherson
2026-09-04 0:24 ` [PATCH v4 17/17] KVM: selftests: Always assert that vmreadz() succeeds Sean Christopherson
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