On Fri, Jan 18, 2019 at 01:20:09PM -0800, Sean Christopherson wrote: > 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@xxxxxxxxxxxxxxx> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> Thank you! > --- > arch/x86/kvm/vmx/vmx.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index 80262c7a4495..23e5fa58751a 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -6401,7 +6401,7 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx) > "mov %%" _ASM_AX", %%cr2 \n\t" > "3: \n\t" > /* Check if vmlaunch or vmresume is needed */ > - "cmpl $0, %c[launched](%%" _ASM_CX ") \n\t" > + "cmpb $0, %c[launched](%%" _ASM_CX ") \n\t" > /* Load guest registers. Don't clobber flags. */ > "mov %c[rax](%%" _ASM_CX "), %%" _ASM_AX " \n\t" > "mov %c[rbx](%%" _ASM_CX "), %%" _ASM_BX " \n\t" > -- > 2.20.1 >
