On Fri, Jan 18, 2019 at 01:20:10PM -0800, Sean Christopherson wrote: > 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@xxxxxxxxxxxxxxx> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> Thank you! > --- > arch/x86/kvm/vmx/nested.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c > index 3170e291215d..69920c2fa5db 100644 > --- a/arch/x86/kvm/vmx/nested.c > +++ b/arch/x86/kvm/vmx/nested.c > @@ -2760,7 +2760,7 @@ 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" > > -- > 2.20.1 >
