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

["Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx>]

Tim Deegan wrote on 2014-11-18:
> At 14:26 +0000 on 18 Nov (1416317209), Ian Campbell wrote:
>> On Tue, 2014-11-18 at 11:41 +0000, Zhang, Yang Z wrote:
>>>> Hmm - this is a pitfall waiting to happen.
>>>> 
>>>> In the case that there is a heterogeneous setup with one 1G
>>>> capable and one 1G incapable server, Xen cannot forcibly prevent
>>>> the use of 1G pages on the capable hardware.  Any VM which
>>>> guesses at hardware support by means other than cpuid features
>>>> is liable to
> explode on migrate.
>>> 
>>> But a normal guest shouldn't to guess it, right?
>> 
>> IMHO any guest which does not use the mechanism explicitly provided
>> for feature detection deserves to break randomly.
> 
> Yes, that's a reasonable position (notwithstanding that we know such
> software exists).
> 

Agree.

> 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).

> 
> Cheers,
> 
> Tim.


Best regards,
Yang


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