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

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Message-Id: <20161019114548.GL3716@linux.vnet.ibm.com>
Date:   Wed, 19 Oct 2016 04:45:48 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        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>, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry

On Sun, Oct 16, 2016 at 05:29:24AM +0300, Michael S. Tsirkin wrote:
> On Sat, Oct 15, 2016 at 03:47:45AM -0400, Paolo Bonzini wrote:
> > 
> > > > 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
> 
> I'm beginning to think that if a value is always (maybe except for init
> where we don't much care about the code size anyway) accessed through
> *_ONCE macros, we should just mark it volatile and be done with it. The
> code will look cleaner, and there will be less space for errors
> like forgetting *_ONCE macros.
> 
> Would such code (where all accesses are done through
> READ_ONCE/WRITE_ONCE otherwise) be an exception to
> volatile-considered-harmful.txt rules?
> 
> Cc Paul and Jonathan (for volatile-considered-harmful.txt).

One concern would be the guy reading the code, to whom it might not
be obvious that the underlying access was volatile, especially if
the reference was a few levels down.

							Thanx, Paul