Re: [Xen-devel] [PATCH] xen: provide pse36 cpuid bit

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 27/10/2011 15:30, "Christoph Egger" <Christoph.Egger@xxxxxxx> wrote:

> On 10/27/11 16:06, Tim Deegan wrote:
>> At 14:10 +0200 on 27 Oct (1319724631), Christoph Egger wrote:
>>> 
>>> Provide pse36 cpuid bit if guest runs in 32bit PAE
>>> or in long mode. Hyper-V refuses to start as
>>> the "cpu does not provide required hw features"
>>> if it does not find the pse36 cpuid bits.
>>> 
>>> Signed-off-by: Christoph Egger<Christoph.Egger@xxxxxxx>
>> 
>> This patch appears to advertise PSE36 support to guests without actually
>> supporting PSE36.  Or am I missing something?
> 
> That's right. The paging format differs only in 32bit legacy mode.
> Since Hyper-V is not running in 32bit legacy mode but insists on having
> these cpuid bits present it is sufficient to just populate them to the
> guest when guest paging mode != 32bit legacy mode.

It would be nice if we didn't have to toggle CPUID.PSE36 based on current
guest mode. How hard would it be to pull out bits 35..32 of a physical
address from bits 16..13 of a legacy 32-bit PDE whose PS flag = 1?

I'm actually surprised we don't do it already, it's so trivial! The code
explicitly says we don't though, and for a reason that makes no sense to
me...

 -- Keir

> Christoph
> 



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