On 04/29/2016 01:34 AM, Eduardo Habkost wrote:
(CCing some additional Intel people)
On Wed, Apr 27, 2016 at 04:13:06PM +0800, Xiao Guangrong wrote:
From: Eduardo Habkost <ehabkost@xxxxxxxxxx>
Introduce Skylake-Client cpu mode which inherits the features from
Broadwell and supports some additional features that are: MPX,
XSAVEC, XSAVES and XGETBV1
Note:
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.
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.
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.
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