linux-kernel - Re: __force_order usage on x86's CRn accesses

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Message-Id: <4FF2E0AE020000780008D48E@nat28.tlf.novell.com>
Date:	Tue, 03 Jul 2012 11:08:14 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Glauber Costa" <glommer@...allels.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>, <jeremy@...p.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: __force_order usage on x86's CRn accesses

>>> On 03.07.12 at 11:55, Glauber Costa <glommer@...allels.com> wrote:
>> Furthermore, the addition of these constraints happened
>> without any real explanation - the code comment that was added
>> doesn't really help understand why "volatile" isn't sufficient here.
> 
> If my memory dont't fail me, I believe this is because gcc will feel
> free to reorder a sequence of instructions that does not access memory.
> Specially since it has no knowledge of what's in the inline assembly,
> and what are its constraints. It only knows that it is an register
> operation, and treats it like one.
> 
> Also, I believe what we are concerned with here is not arbitrary reorder
> between that and other instructions, which we welcome, but reordering
> between a read and a write to the same crX - specially of concern for
> things doing read-modify-writes of control registers.

But such sequences can't be re-ordered when the asm-s are
volatile qualified.

>> Finally (and this is because I lack the explanation why the
>> artificial constraint is needed in the first place), why is it that
>> clts() doesn't need one too?
>> 
> 
> Because we're not using it to do read-modify-write of the control register.

Sequences of read_cr0()/clts()/write_cr0() certainly exist (in
the xor implementations), so if a read_cr0()/write_cr0() can
have issues (which is impossible if the value written depends on
the one read), read_cr0()/clts() would be even more affected,
as there's no compiler visible dependency between the two.

But I don't think any such can happen in reality, and I was
hoping to be provided with an example of proving me wrong
(and understand the whole situation).

Jan

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