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Re: [Qemu-devel] [PATCH] target-i386: add Skylake-Client cpu mode

From: Xiao Guangrong
Subject: Re: [Qemu-devel] [PATCH] target-i386: add Skylake-Client cpu mode
Date: Tue, 3 May 2016 14:38:44 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

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 <address@hidden>

Introduce Skylake-Client cpu mode which inherits the features from
Broadwell and supports some additional features that are: MPX,

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 &

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 &= 
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 

During boot, the capability is adjusted by cpu_caps_cleared, in 
 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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