KVM: nVMX: Check a single byte for VMCS "launched" in nested early checks

Nested early checks does a manual comparison of a VMCS' launched status
in its asm blob 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].

The use of 'cmpl' instead of 'cmpb' was not deliberate, but rather the
result of a copy-paste as the asm blob was directly derived from the asm
blob for vCPU-run.

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: 52017608da33 ("KVM: nVMX: add option to perform early consistency checks via H/W")
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>

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index d8ea4ebd79e787050e0adfd861bb3df56aa3ec66..50c9e04910ea6b52a954bf8e856fcad9e25b346a 100644 (file)
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2761,7 +2761,7 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
                "add $%c[wordsize], %%" _ASM_SP "\n\t" /* un-adjust RSP */
 
                /* Check if vmlaunch or vmresume is needed */
-               "cmpl $0, %c[launched](%% " _ASM_CX")\n\t"
+               "cmpb $0, %c[launched](%% " _ASM_CX")\n\t"
 
                "call vmx_vmenter\n\t"