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

From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH] target-i386: add Skylake-Client cpu mode
Date: Thu, 28 Apr 2016 14:34:43 -0300
User-agent: Mutt/1.5.24 (2015-08-30)

(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,
> 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 &

Is GET_SUPPORTED_CPUID returning XSAVES as unsupported in the
system where you are running tests?


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