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

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Date:   Sat, 15 Oct 2016 03:47:45 -0400 (EDT)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Nadav Amit <nadav.amit@...il.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.

There is no documentation for this in the kernel tree unfortunately.
But yes, I think we should do that.  Using READ_ONCE/WRITE_ONCE around
memory barriers is a start.

Paolo