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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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).

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...)

Ian.


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