linux-kernel - Re: [RFC PATCH 0/3] kvm: x86: speedups for APICv

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Date:   Fri, 30 Sep 2016 15:33:23 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        yang zhang wz <yang.zhang.wz@...il.com>,
        feng wu <feng.wu@...el.com>, mst@...hat.com
Subject: Re: [RFC PATCH 0/3] kvm: x86: speedups for APICv

2016-09-29 17:41-0400, Paolo Bonzini:
>> And a more far-fetched one: if we know that PI.ON is set before vm
>> entry, we could just send POSTED_INTR_VECTOR self-IPI after masking
>> interrupts and let APICv copy PIR to IRR and deliver interrupts.
>> There are two possible drawbacks: Is the self-IPI overhead too big?
>> Would APICv IRR evaluation at vm entry take precedence, so we'd have big
>> interrupt priority inversion window?
> 
> I don't think there is a risk of inverting interrupt priority, because
> that race is always present.  But the overhead is probably too much, the
> cost of the one xchg in __apic_update_irr is probably half of the whole
> IRR update if the PI descriptor cacheline bounces.

Yep, I just ran the vmexit kvm-unit-benchmark -- the cpuid and hypercall
tests are ~1000 cycles slower if I send the notification self-IPI, which
should be far more than PIR->IRR + vmcs_write(RVI, fls(IRR)).