Re: [PATCH] KVM: x86: fix kvm_vcpu_is_preempted

[Paolo Bonzini <pbonzini@redhat.com>]

On 1/12/22 12:19, 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

The combination of PREEMPTED=0,FLUSH_TLB=1 is invalid and can only 
happens if the guest malfunctions (which it doesn't, it uses cmpxchg to 
set KVM_VCPU_PREEMPTED); the host only does an xchg with 0 as the new 
value.  Since this is guest code, this patch does not change an actual 
error in the code, does it?

Paolo

> Fixes: 858a43aae2367 ("KVM: X86: use paravirtualized TLB Shootdown")
> Signed-off-by: Li RongQing<lirongqing@baidu.com>
> ---
>   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;"
>   "ret;"
>   ".size __raw_callee_save___kvm_vcpu_is_preempted, .-__raw_callee_save___kvm_vcpu_is_preempted;"
>   ".popsection");