Re: [Xen-devel] Xen hypervisor hangs on booting against c/s 16925

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 29.01.08 11:54 >>>
>On 29/1/08 10:52, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:
>
>>> If we don't write to a MTRR WP region, then actually mapping as WC does not
>>> matter. We never write to the range 0xc0000-0xfffff. Anything we write to
>>> below 0xa0000 had better be RAM, or we've got bigger problems than the WP
>>> attribute!
>> 
>> Unfortunately it does matter - WC does not permit read caching, but
>> does permit speculative reads. That's both not what we want.
>
>Is your concern that something with read side effects (i.e., I/O) would be
>mapped with WP type in the MTRR? Does that not seem rather unlikely?

No, it was rather the ordering of reads that I was concerned about, but ...

>Anyhow, WP attribute also allows read speculation, just like WC, so in
>respect of read speculation it does not matter which caching attribute we
>actually use.

... yes, I didn't look closely enough at the WP characteristics. So we
would only inhibit read caching then.

Jan


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