linux-kernel - Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in function pvclock_read

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Date:	Thu, 9 Jun 2016 14:31:15 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Borislav Petkov <bp@...e.de>
Cc:	Minfei Huang <mnghuan@...il.com>, luto@...nel.org, hpa@...or.com,
	mingo@...hat.com, tglx@...utronix.de, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in
 function pvclock_read_flags

On 09/06/2016 13:28, Borislav Petkov wrote:
>> > technically, they should be virt_rmb() -- it really doesn't matter of
>> > course because reads are never reordered on x86.
> You mean
> 
> 	version = src->version;
> 	flags = src->flags;
> 
> are not reordered?
> 
> I don't think so.

The compiler can reorder them, so smp_rmb() and virt_rmb() have to be
barrier(), but the processor won't.

x86 will only move a store after a subsequent load, if you exclude
special cases such as write combining, non-temporal moves and the like.
LFENCE and SFENCE are only needed for those special cases.

Paolo