| Message ID | 1584185480-3556-1-git-send-email-pbonzini@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: X86: correct meaningless kvm_apicv_activated() check | expand |
Paolo Bonzini <pbonzini@redhat.com> writes: > After test_and_set_bit() for kvm->arch.apicv_inhibit_reasons, we will > always get false when calling kvm_apicv_activated() because it's sure > apicv_inhibit_reasons do not equal to 0. > > What the code wants to do, is check whether APICv was *already* active > and if so skip the costly request; we can do this using cmpxchg. > > Reported-by: Miaohe Lin <linmiaohe@huawei.com> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> > --- > arch/x86/kvm/x86.c | 25 ++++++++++++++++--------- > 1 file changed, 16 insertions(+), 9 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index a7cb85231330..49efa4529662 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -8049,19 +8049,26 @@ void kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu) > */ > void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit) > { > + unsigned long old, new, expected; > + > if (!kvm_x86_ops->check_apicv_inhibit_reasons || > !kvm_x86_ops->check_apicv_inhibit_reasons(bit)) > return; > > - if (activate) { > - if (!test_and_clear_bit(bit, &kvm->arch.apicv_inhibit_reasons) || > - !kvm_apicv_activated(kvm)) > - return; > - } else { > - if (test_and_set_bit(bit, &kvm->arch.apicv_inhibit_reasons) || > - kvm_apicv_activated(kvm)) > - return; > - } > + old = READ_ONCE(kvm->arch.apicv_inhibit_reasons); > + do { > + expected = new = old; > + if (activate) > + __clear_bit(bit, &new); > + else > + __set_bit(bit, &new); > + if (new == old) > + break; > + old = cmpxchg(&kvm->arch.apicv_inhibit_reasons, expected, new); > + } while (old != expected); 'expected' here is a bit confusing as it's not what we expect to get as the result but rather what we expect to see pre-change. I don't have a better suggestion though. > + > + if ((old == 0) == (new == 0)) > + return; This is a very laconic expression I personally find hard to read :-) /* Check if WE actually changed APICv state */ if ((!old && !new) || (old && new)) return; would be my preference (not strong though, I read yours several times and now I feel like I understand it just fine :-) > > trace_kvm_apicv_update_request(activate, bit); > if (kvm_x86_ops->pre_update_apicv_exec_ctrl) Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
On Mon, Mar 16, 2020 at 09:33:50AM +0100, Vitaly Kuznetsov wrote: > Paolo Bonzini <pbonzini@redhat.com> writes: > > + if ((old == 0) == (new == 0)) > > + return; > > This is a very laconic expression I personally find hard to read :-) > > /* Check if WE actually changed APICv state */ > if ((!old && !new) || (old && new)) > return; > > would be my preference (not strong though, I read yours several times > and now I feel like I understand it just fine :-) Or maybe this to avoid so many equals signs? if (!old == !new) return;
Sean Christopherson <sean.j.christopherson@intel.com> writes: > On Mon, Mar 16, 2020 at 09:33:50AM +0100, Vitaly Kuznetsov wrote: >> Paolo Bonzini <pbonzini@redhat.com> writes: >> > + if ((old == 0) == (new == 0)) >> > + return; >> >> This is a very laconic expression I personally find hard to read :-) >> >> /* Check if WE actually changed APICv state */ >> if ((!old && !new) || (old && new)) >> return; >> >> would be my preference (not strong though, I read yours several times >> and now I feel like I understand it just fine :-) > > Or maybe this to avoid so many equals signs? > > if (!old == !new) > return; > if (!!old == !!new) return; to make it clear we're converting them to 1/0 :-)
On Mon, Mar 16, 2020 at 04:44:47PM +0100, Vitaly Kuznetsov wrote: > Sean Christopherson <sean.j.christopherson@intel.com> writes: > > > On Mon, Mar 16, 2020 at 09:33:50AM +0100, Vitaly Kuznetsov wrote: > >> Paolo Bonzini <pbonzini@redhat.com> writes: > >> > + if ((old == 0) == (new == 0)) > >> > + return; > >> > >> This is a very laconic expression I personally find hard to read :-) > >> > >> /* Check if WE actually changed APICv state */ > >> if ((!old && !new) || (old && new)) > >> return; > >> > >> would be my preference (not strong though, I read yours several times > >> and now I feel like I understand it just fine :-) > > > > Or maybe this to avoid so many equals signs? > > > > if (!old == !new) > > return; > > > > if (!!old == !!new) > return; > > to make it clear we're converting them to 1/0 :-) All I can think of now is the Onion article regarding razor blades... if (!!!!old == !!!!new) return;
On 16/03/20 16:59, Sean Christopherson wrote: >>> >> if (!!old == !!new) >> return; >> >> to make it clear we're converting them to 1/0 :-) > > All I can think of now is the Onion article regarding razor blades... > > if (!!!!old == !!!!new) > return; > That would be !!!!!, but seriously I'll go with two. (Thanks for giving me a chuckle, it's sorely needed these days). Paolo
On 3/17/2020 12:39 AM, Paolo Bonzini wrote: > On 16/03/20 16:59, Sean Christopherson wrote: >>>> >>> if (!!old == !!new) >>> return; >>> >>> to make it clear we're converting them to 1/0 :-) >> >> All I can think of now is the Onion article regarding razor blades... >> >> if (!!!!old == !!!!new) >> return; >> > > That would be !!!!!, but seriously I'll go with two. > > (Thanks for giving me a chuckle, it's sorely needed these days). Take care, Paolo. I have been staying at home for two months in Wuhan, China, and things are going better now. I believe all the world can defeat Coronavirus eventually. > Paolo >
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index a7cb85231330..49efa4529662 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8049,19 +8049,26 @@ void kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu) */ void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit) { + unsigned long old, new, expected; + if (!kvm_x86_ops->check_apicv_inhibit_reasons || !kvm_x86_ops->check_apicv_inhibit_reasons(bit)) return; - if (activate) { - if (!test_and_clear_bit(bit, &kvm->arch.apicv_inhibit_reasons) || - !kvm_apicv_activated(kvm)) - return; - } else { - if (test_and_set_bit(bit, &kvm->arch.apicv_inhibit_reasons) || - kvm_apicv_activated(kvm)) - return; - } + old = READ_ONCE(kvm->arch.apicv_inhibit_reasons); + do { + expected = new = old; + if (activate) + __clear_bit(bit, &new); + else + __set_bit(bit, &new); + if (new == old) + break; + old = cmpxchg(&kvm->arch.apicv_inhibit_reasons, expected, new); + } while (old != expected); + + if ((old == 0) == (new == 0)) + return; trace_kvm_apicv_update_request(activate, bit); if (kvm_x86_ops->pre_update_apicv_exec_ctrl)
After test_and_set_bit() for kvm->arch.apicv_inhibit_reasons, we will always get false when calling kvm_apicv_activated() because it's sure apicv_inhibit_reasons do not equal to 0. What the code wants to do, is check whether APICv was *already* active and if so skip the costly request; we can do this using cmpxchg. Reported-by: Miaohe Lin <linmiaohe@huawei.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> --- arch/x86/kvm/x86.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-)