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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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