On Thu, Jan 24, 2019 at 9:59 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> Unlike the primary vCPU-run flow, the nested early checks code doesn't
> actually want to propagate VM-Fail back 'vmx'. Yay copy+paste.
>
> In additional to eliminating the need to clear vmx->fail before
> returning, using a local boolean also drops a reference to 'vmx' in the
> asm blob. Dropping the reference to 'vmx' will save a register in the
> long run as future patches will shift all pointer references from 'vmx'
> to 'vmx->loaded_vmcs'.
>
> Fixes: 52017608da33 ("KVM: nVMX: add option to perform early consistency checks via H/W")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
> arch/x86/kvm/vmx/nested.c | 16 ++++++++++------
> 1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index b92291b8e54b..495edb36b525 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2717,6 +2717,7 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> unsigned long cr3, cr4;
> + bool vm_fail;
>
> if (!nested_early_check)
> return 0;
> @@ -2762,14 +2763,18 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
> /* Check if vmlaunch or vmresume is needed */
> "cmpb $0, %c[launched](%% " _ASM_CX")\n\t"
>
> + /*
> + * VMLAUNCH and VMRESUME clear RFLAGS.{CF,ZF} on VM-Exit, set
> + * RFLAGS.CF on VM-Fail Invalid and set RFLAGS.ZF on VM-Fail
> + * Valid. vmx_vmenter() directly "returns" RFLAGS, and so the
> + * results of VM-Enter is captured via SETBE to vm_fail.
> + */
> "call vmx_vmenter\n\t"
>
> - /* Set vmx->fail accordingly */
> - "setbe %c[fail](%% " _ASM_CX")\n\t"
> - : ASM_CALL_CONSTRAINT
> + "setbe %%dl\n\t"
> + : ASM_CALL_CONSTRAINT, "=dl"(vm_fail)
Why hardcode %dl? Can't you use an '=q' constraint and let the compiler choose?
