Re: [Xen-devel] [PATCH] libxc: Expose the pdpe1gb cpuid flag to guest

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.11.14 at 02:47, <yang.z.zhang@xxxxxxxxx> wrote:
> Tim Deegan wrote on 2014-11-19:
>> At 01:29 +0000 on 19 Nov (1416356943), Zhang, Yang Z wrote:
>>> Tim Deegan wrote on 2014-11-18:
>>>> In this case, the guest is entitled to _expect_ pagefaults on 1GB
>>>> mappings if CPUID claims they are not supported.  That sounds like
>>>> an unlikely thing for the guest to be relying on, but Xen itself
>>>> does something similar for the SHOPT_FAST_FAULT_PATH (and now also
>>>> for IOMMU entries for the deferred caching attribute updates).
>>> 
>>> Indeed. How about adding the software check (as Andrew mentioned)
>>> firstly and leave the hardware problem (Actually, I don't think we
>>> can solve it currently).
>> 
>> I don't think we should change the software path unless we can change
>> the hardware behaviour too.  It's better to be consistent, and it
>> saves us some cycles in the pt walker.
> 
> So if we don't need to add the software check, does this mean current patch 
> is ok?

No - Tim having confirmed that shadow mode doesn't support 1Gb
pages, the feature clearly must not be made visible for shadow
mode guest.

Jan


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