linux-kernel - Re: [PATCH] KVM: X86: avoid meaningless kvm_apicv

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Message-ID: <87d0a2n8g9.fsf@vitty.brq.redhat.com>
Date:   Tue, 25 Feb 2020 13:43:34 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        pbonzini@...hat.com, rkrcmar@...hat.com,
        sean.j.christopherson@...el.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com
Subject: Re: [PATCH] KVM: X86: avoid meaningless kvm_apicv_activated() check

linmiaohe <linmiaohe@...wei.com> writes:

> From: Miaohe Lin <linmiaohe@...wei.com>
>
> 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.
>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  arch/x86/kvm/x86.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ddcc51b89e2c..fa62dcb0ed0c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8018,8 +8018,7 @@ void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
>  		    !kvm_apicv_activated(kvm))
>  			return;
>  	} else {
> -		if (test_and_set_bit(bit, &kvm->arch.apicv_inhibit_reasons) ||
> -		    kvm_apicv_activated(kvm))
> +		if (test_and_set_bit(bit, &kvm->arch.apicv_inhibit_reasons))
>  			return;
>  	}

This seems to be correct in a sense that we are not really protected
against concurrent modifications of 'apicv_inhibit_reasons' (like what
if 'apicv_inhibit_reasons' gets modified right after we've checked
'kvm_apicv_activated(kvm)').

The function, however, still gives a flase impression it is somewhat
protected against concurent modifications. Like what are these
test_and_{set,clear}_bit() for?

If I'm not mistaken, the logic this function was supposed to implement
is: change the requested bit to the requested state and, if
kvm_apicv_activated() changed (we set the first bit or cleared the
last), proceed with KVM_REQ_APICV_UPDATE. What if we re-write it like

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2103101eca78..b97b8ff4a789 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8027,19 +8027,19 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_update_apicv);
  */
 void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
 {
+       bool apicv_was_activated = kvm_apicv_activated(kvm);
+
        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;
-       }
+       if (activate)
+               clear_bit(bit, &kvm->arch.apicv_inhibit_reasons);
+       else
+               set_bit(bit, &kvm->arch.apicv_inhibit_reasons);
+
+       if (kvm_apicv_activated(kvm) == apicv_was_activated)
+               return;
 
        trace_kvm_apicv_update_request(activate, bit);
        if (kvm_x86_ops->pre_update_apicv_exec_ctrl)

Is this equal?

-- 
Vitaly