Re: [PATCH 3/16] read/write_crX, clts and wbinvd for 64-bit paravirt

Glauber de Oliveira Costa
2007-11-01T16:13:55Z

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Keir Fraser escreveu:
> On 1/11/07 15:30, "Jeremy Fitzhardinge" <jeremy-TSDbQ3PG+2Y@public.gmane.org> wrote:
>
>> Glauber de Oliveira Costa wrote:
>>> I in fact have seen bugs with mixed reads and writes to the same cr,
>>> (cr4), but adding the volatile
>>> flag to the read function seemed to fix it.
>> Well, volatile will make a read be repeated rather than caching the
>> previous value, but it has no effect on ordering.
>
> volatile prevents the asm from being 'moved significantly', according to the
> gcc manual. I take that to mean that reordering is not allowed.
>
According to a gcc developer to whom I asked this question, volatile
prevents the code
to be removed, but does not prevent it to be moved (pun indented). In
practice, it should force
a re-read, but not influence the ordering decisions from the compiler.
Besides , 'significantly'
sounds like a significantly unprecise word, whose specific meaning may
be implementation dependant.

So I agree that adding a memory location reference is probably the best
alternative.

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