(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
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