Re: [Xen-devel] 1GB hugepages and intel_xc_cpuid_policy by default disables it.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.01.14 at 11:10, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Fri, 2014-01-10 at 13:41 -0500, Konrad Rzeszutek Wilk wrote:
>> and this is the only way to get hugepages working
>> (for reference here is what I have in the guest command line:
>> "default_hugepagesz=1G hugepagesz=1G hugepages=2 hugepagesz=2M
>> hugepages=100" and without this cpuid I get:
>>  hugepagesz: Unsupported page size 1024 M 
>> 
>> because the 1GB flag is not exposed.
>> 
>> I was wondering why we don't check the host cpuid and set the 1GB
>> flag (pdpe1gb) for those cases? It seems that all the pieces are in
>> place for this to work?
> 
> The hypervisor currently contains :
>         /* Hide 1GB-superpage feature if we can't emulate it. */
>         if (!hvm_pse1gb_supported(d))
>             *edx &= ~cpufeat_mask(X86_FEATURE_PAGE1GB);
> 
> So I think it would be safe to enable this by default in userspace and
> let the hypervisor clear it if unsafe. But TBH the whole cpuid policy is
> a mystery to me. Can you cc some likely hypervisor side people on your
> patch for confirmation please.

In fact I can't see where this would be forced off: xc_cpuid_x86.c
only does so in the PV case, and all hvm_pse1gb_supported() is
that the CPU supports it and the domain uses HAP.

Jan


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