linux-kernel - Re: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry

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Message-Id: <9B7F4808-2294-426D-B463-CEB188CED2E0@gmail.com>
Date:   Fri, 14 Oct 2016 12:44:53 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Yang Zhang <yang.zhang.wz@...il.com>,
        feng wu <feng.wu@...el.com>, mst@...hat.com
Subject: Re: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry


> On Oct 14, 2016, at 11:56 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>> 
>>> 	for (i = 0; i <= 7; i++) {
>>> -		pir_val = xchg(&pir[i], 0);
>>> -		if (pir_val)
>>> +		pir_val = READ_ONCE(pir[i]);
>> 
>> Out of curiosity, do you really need this READ_ONCE?
> 
> The answer can only be "depends on the compiler's whims". :)
> If you think of READ_ONCE as a C11 relaxed atomic load, then yes.

Hm.. So the idea is to make the code "race-free” in the sense
that every concurrent memory access is done using READ_ONCE/WRITE_ONCE?

If that is the case, I think there are many other cases that need to be
changed, for example apic->irr_pending and vcpu->arch.pv.pv_unhalted.