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

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

Konrad Rzeszutek Wilk wrote on 2014-11-10:
> On Mon, Nov 10, 2014 at 05:08:09AM +0000, Zhang, Yang Z wrote:
>> Konrad Rzeszutek Wilk wrote on 2014-01-16:
>>> On Mon, Jan 13, 2014 at 11:51:28AM +0000, Jan Beulich wrote:
>>>>>>> On 13.01.14 at 12:38, Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> wrote:
>>>>> On Mon, 2014-01-13 at 11:30 +0000, Jan Beulich wrote:
>>>>>> 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.
>>>>> 
>>>>> Took me a while to spot it too:
>>>>> static void intel_xc_cpuid_policy( [...]
>>>>>             case 0x80000001: {
>>>>>                 int is_64bit = hypervisor_is_64bit(xch) &&
>>>>> is_pae;
>>>>> 
>>>>>                 /* Only a few features are advertised in Intel's
>>>>>                 0x80000001. */ regs[2] &= (is_64bit ?
>>> bitmaskof(X86_FEATURE_LAHF_LM) : 0) |
>>>>> 
>>> bitmaskof(X86_FEATURE_ABM);
>>>>>                 regs[3] &= ((is_pae ? bitmaskof(X86_FEATURE_NX) :
>>>>> 0)
>>> |
>>>>>                             (is_64bit ? bitmaskof(X86_FEATURE_LM) :
>>>>>                             0) | (is_64bit ?
>>>>>                             bitmaskof(X86_FEATURE_SYSCALL) : 0) |
>>>>>                             (is_64bit ?
>>> bitmaskof(X86_FEATURE_RDTSCP) : 0));
>>>>>                 break;
>>>>>             }
>>>>> Which masks anything which is not explicitly mentioned. (PAGE1GB
>>>>> is in regs[3], I think).
>>>> 
>>>> Ah, okay. The funs of white listing on HVM vs black listing on PV
>>>> again.
>>>> 
>>>>> The AMD version is more permissive:
>>>>> 
>>>>>         regs[3] &= (0x0183f3ff | /* features shared with
>>> 0x00000001:EDX */
>>>>>                     (is_pae ? bitmaskof(X86_FEATURE_NX) : 0) |
>>>>>                     (is_64bit ? bitmaskof(X86_FEATURE_LM) : 0) |
>>>>>                     bitmaskof(X86_FEATURE_SYSCALL) |
>>>>>                     bitmaskof(X86_FEATURE_MP) |
>>>>>                     bitmaskof(X86_FEATURE_MMXEXT) |
>>>>>                     bitmaskof(X86_FEATURE_FFXSR) |
>>>>>                     bitmaskof(X86_FEATURE_3DNOW) |
>>>>>                     bitmaskof(X86_FEATURE_3DNOWEXT)); (but I
>>>>> didn't check if PAGE1GB is in that magic number...)
>>>> 
>>>> It's not - it's bit 26.
>>> 
>>> So.. it sounds to me like everybody is in the agreement that this
>>> is the right thing to do (enable it if the hypervisor has it enabled)?
>>> 
>>> And the next thing is actually come up with a patch to do some of
>>> this plumbing - naturally for Xen 4.5?
>> 
>> Hi, Konrad,
>> 
>> Is there any patch to turn on the 1GB hugepages? If no, we are happy
>> to give
> a patch to do it.
> 
> I have not see a patch for this, and I would be quite happy to see
> patch developed for this!

OK. We will provide a patch ASAP.

Best regards,
Yang



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