> -----邮件原件-----
> 发件人: Sean Christopherson <seanjc@xxxxxxxxxx>
> 发送时间: 2022年1月13日 0:44
> 收件人: Li,Rongqing <lirongqing@xxxxxxxxx>
> 抄送: pbonzini@xxxxxxxxxx; vkuznets@xxxxxxxxxx; wanpengli@xxxxxxxxxxx;
> jmattson@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; bp@xxxxxxxxx;
> x86@xxxxxxxxxx; hpa@xxxxxxxxx; rkrcmar@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> joro@xxxxxxxxxx
> 主题: Re: [PATCH] KVM: x86: fix kvm_vcpu_is_preempted
>
> 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.
>
Build with gcc (GCC) 7.3.0, disassembled as blow
before:
ffffffff8105bdc0 <__raw_callee_save___kvm_vcpu_is_preempted>:
ffffffff8105bdc0: 48 8b 04 fd c0 16 1d mov -0x7de2e940(,%rdi,8),%rax
ffffffff8105bdc7: 82
ffffffff8105bdc8: 80 b8 90 c0 02 00 00 cmpb $0x0,0x2c090(%rax)
ffffffff8105bdcf: 0f 95 c0 setne %al
ffffffff8105bdd2: c3 retq
ffffffff8105bdd3: 0f 1f 00 nopl (%rax)
ffffffff8105bdd6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
ffffffff8105bddd: 00 00 00
after:
ffffffff8105bdc0 <__raw_callee_save___kvm_vcpu_is_preempted>:
ffffffff8105bdc0: 48 8b 04 fd c0 16 1d mov -0x7de2e940(,%rdi,8),%rax
ffffffff8105bdc7: 82
ffffffff8105bdc8: 8a 80 90 c0 02 00 mov 0x2c090(%rax),%al
ffffffff8105bdce: 24 01 and $0x1,%al
ffffffff8105bdd0: c3 retq
ffffffff8105bdd1: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
ffffffff8105bdd6: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
ffffffff8105bddd: 00 00 00
Thanks
-Li
