[ Upstream commit 61c08aa9606d4e48a8a50639c956448a720174c3 ]
The vCPU-run asm blob does a manual comparison of a VMCS' launched status to execute the correct VM-Enter instruction, i.e. VMLAUNCH vs. VMRESUME. The launched flag is a bool, which is a typedef of _Bool. C99 does not define an exact size for _Bool, stating only that is must be large enough to hold '0' and '1'. Most, if not all, compilers use a single byte for _Bool, including gcc[1].
Originally, 'launched' was of type 'int' and so the asm blob used 'cmpl' to check the launch status. When 'launched' was moved to be stored on a per-VMCS basis, struct vcpu_vmx's "temporary" __launched flag was added in order to avoid having to pass the current VMCS into the asm blob. The new '__launched' was defined as a 'bool' and not an 'int', but the 'cmp' instruction was not updated.
This has not caused any known problems, likely due to compilers aligning variables to 4-byte or 8-byte boundaries and KVM zeroing out struct vcpu_vmx during allocation. I.e. vCPU-run accesses "junk" data, it just happens to always be zero and so doesn't affect the result.
[1] https://gcc.gnu.org/ml/gcc-patches/2000-10/msg01127.html
Fixes: d462b8192368 ("KVM: VMX: Keep list of loaded VMCSs, instead of vcpus") Cc: stable@vger.kernel.org Reviewed-by: Jim Mattson jmattson@google.com Reviewed-by: Konrad Rzeszutek Wilk konrad.wilk@oracle.com Signed-off-by: Sean Christopherson sean.j.christopherson@intel.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kvm/vmx.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index 2e310ea62d609..562f5dc4645b6 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -10808,7 +10808,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu) "mov %%" _ASM_AX", %%cr2 \n\t" "3: \n\t" /* Check if vmlaunch of vmresume is needed */ - "cmpl $0, %c[launched](%0) \n\t" + "cmpb $0, %c[launched](%0) \n\t" /* Load guest registers. Don't clobber flags. */ "mov %c[rax](%0), %%" _ASM_AX " \n\t" "mov %c[rbx](%0), %%" _ASM_BX " \n\t"