[Qemu-devel] XSAVES in GET_SUPPORTED_CPUID (was Re: [PATCH] target-i386: add Skylake-Client cpu mode)

[Eduardo Habkost]

(CCing KVM mailing list)

On Tue, May 03, 2016 at 02:38:44PM +0800, Xiao Guangrong wrote:
> On 04/29/2016 01:34 AM, Eduardo Habkost wrote:
> >On Wed, Apr 27, 2016 at 04:13:06PM +0800, Xiao Guangrong wrote:
[...]
> >>1. As XSAVES is disabled in upstream linux kernel by commit e88221c50
> >>(x86/fpu: Disable XSAVES* support for now), QEMU will complain about
> >>"warning: host doesn't support requested feature: CPUID.0DH:EAX.xsaves [bit 
> >>3]"
> >
> >I have been looking at the GET_SUPPORTED_CPUID code and I am not
> >sure if commit e88221c50 is supposed to be affecting
> >GET_SUPPORTED_CPUID, or not. It looks like it shouldn't, so I
> >don't know why QEMU is reporting xsaves as unsupported.
> >
> >For reference, GET_SUPPORTED_CPUID code for function == 0xd &&
> >idx == 1 will run:
> >
> >   unsigned f_xsaves = kvm_x86_ops->xsaves_supported() ? F(XSAVES) : 0;
> >   const u32 kvm_cpuid_D_1_eax_x86_features =
> >           F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | f_xsaves;
> >   /* [...] */
> >   do_cpuid_1_ent(&entry[i], function, idx);
> >   entry[i].eax &= kvm_cpuid_D_1_eax_x86_features;
> >
> >do_cpuid_1_ent() just executes the CPUID instruction.
> >
> >kvm_x86_ops->xsaves_supported is:
> >
> >   static bool vmx_xsaves_supported(void)
> >   {
> >           return vmcs_config.cpu_based_2nd_exec_ctrl &
> >                   SECONDARY_EXEC_XSAVES;
> >   }
> >
> >Is GET_SUPPORTED_CPUID returning XSAVES as unsupported in the
> >system where you are running tests?
> 
> No, it returns that XSAVES is supported.

You mean it returns it as unsupported, right?

> 
> Actually F(SAVES) bit is cleared later, in __do_cpuid_ent():
> 536                                 entry[i].eax &= 
> kvm_cpuid_D_1_eax_x86_features;
> 537                                 cpuid_mask(&entry[i].eax, CPUID_D_1_EAX);
> 
> The bits unsupported by boot_cpu_data.x86_capability[CPUID_D_1_EAX] will be 
> cleared.

Oh, I didn't notice the cpuid_mask() call. You're right.

> 
> During boot, the capability is adjusted by cpu_caps_cleared, in 
> arch/x86/kernel/cpu/common.c:
>  971         /* Clear/Set all flags overriden by options, after probe */
>  972         for (i = 0; i < NCAPINTS; i++) {
>  973                 c->x86_capability[i] &= ~cpu_caps_cleared[i];
>  974                 c->x86_capability[i] |= cpu_caps_set[i];
>  975         }
> 
> Actually, setup_clear_cpu_cap() exactly acts on cpu_caps_cleared():
> 112 #define setup_clear_cpu_cap(bit) do { \
> 113         clear_cpu_cap(&boot_cpu_data, bit);     \
> 114         set_bit(bit, (unsigned long *)cpu_caps_cleared); \
> 115 } while (0)
> 
> This is the reason why setup_clear_cpu_cap(X86_FEATURE_XSAVES) introduced by 
> commit e88221c50
> caused XSAVES unreported by KVM.

So, is this the right behavior, or KVM can support exposing
XSAVES to guests even if the cpu_cap bit is cleared? I don't know
if exposing XSAVES to KVM guests depend on reworking kernel xsave
code or not.

-- 
Eduardo