On Wed, Jan 12, 2022 at 04:43:39PM +0000, Sean Christopherson wrote:
> On Wed, Jan 12, 2022, Li RongQing wrote:
> > After support paravirtualized TLB shootdowns, steal_time.preempted
> > includes not only KVM_VCPU_PREEMPTED, but also KVM_VCPU_FLUSH_TLB
> >
> > and kvm_vcpu_is_preempted should test only with KVM_VCPU_PREEMPTED
> >
> > Fixes: 858a43aae2367 ("KVM: X86: use paravirtualized TLB Shootdown")
> > Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> > ---
> > arch/x86/kernel/kvm.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index 59abbda..a9202d9 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -1025,8 +1025,8 @@ asm(
> > ".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
> > "__raw_callee_save___kvm_vcpu_is_preempted:"
> > "movq __per_cpu_offset(,%rdi,8), %rax;"
> > -"cmpb $0, " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax);"
> > -"setne %al;"
> > +"movb " __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax), %al;"
> > +"andb $" __stringify(KVM_VCPU_PREEMPTED) ", %al;"
>
> Eww, the existing code is sketchy. It relies on the compiler to store _Bool/bool
> in a single byte since %rax may be non-zero from the __per_cpu_offset(), and
> modifying %al doesn't zero %rax[63:8]. I doubt gcc or clang use anything but a
> single byte on x86-64, but "andl" is just as cheap so I don't see any harm in
> being paranoid.
Agreed, better to clear the rest of rax just to be safe.
