[Devel] Re: [PATCH] mark read_crX() asm code as volatile

[Chuck Ebbert]

On 10/02/2007 11:28 AM, Arjan van de Ven wrote:
> On Tue, 02 Oct 2007 18:08:32 +0400
> Kirill Korotaev <dev at openvz.org> wrote:
> 
>> Some gcc versions (I checked at least 4.1.1 from RHEL5 & 4.1.2 from
>> gentoo) can generate incorrect code with read_crX()/write_crX()
>> functions mix up, due to cached results of read_crX().
>>
> 
> I'm not so sure volatile is the right answer, as compared to giving the
> asm more strict contraints....
> 
> asm volatile tends to mean something else than "the result has
> changed"....

It means "don't eliminate this code if it's reachable" which should be
just enough for this case. But it could still be reordered in some cases
that could break, I think.

This should work because the result gets used before reading again:

read_cr3(a);
write_cr3(a | 1);
read_cr3(a);

But this might be reordered so that b gets read before the write:

read_cr3(a);
write_cr3(a | 1);
read_cr3(b);

?