linux-kernel - Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves

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Date:   Thu, 20 Jun 2019 14:46:50 +0800
From:   Xiaoyao Li <xiaoyao.li@...ux.intel.com>
To:     Wanpeng Li <kernellwp@...il.com>, Tao Xu <tao3.xu@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves_supported

On 6/20/2019 2:40 PM, Wanpeng Li wrote:
> Hi,
> On Thu, 20 Jun 2019 at 13:06, Tao Xu <tao3.xu@...el.com> wrote:
>>
>> The helper vmx_xsaves_supported() returns the bit value of
>> SECONDARY_EXEC_XSAVES in vmcs_config.cpu_based_2nd_exec_ctrl, which
>> remains unchanged true if vmcs supports 1-setting of this bit after
>> setup_vmcs_config(). It should check the guest's cpuid not this
>> unchanged value when get/set msr.
>>
>> Besides, vmx_compute_secondary_exec_control() adjusts
>> SECONDARY_EXEC_XSAVES bit based on guest cpuid's X86_FEATURE_XSAVE
>> and X86_FEATURE_XSAVES, it should use updated value to decide whether
>> set XSS_EXIT_BITMAP.
>>
>> Co-developed-by: Xiaoyao Li <xiaoyao.li@...ux.intel.com>
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...ux.intel.com>
>> Signed-off-by: Tao Xu <tao3.xu@...el.com>
>> ---
>>   arch/x86/kvm/vmx/vmx.c | 8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index b93e36ddee5e..935cf72439a9 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -1721,7 +1721,8 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>                  return vmx_get_vmx_msr(&vmx->nested.msrs, msr_info->index,
>>                                         &msr_info->data);
>>          case MSR_IA32_XSS:
>> -               if (!vmx_xsaves_supported())
>> +               if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) ||
>> +                       !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
>>                          return 1;
>>                  msr_info->data = vcpu->arch.ia32_xss;
>>                  break;
>> @@ -1935,7 +1936,8 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>                          return 1;
>>                  return vmx_set_vmx_msr(vcpu, msr_index, data);
>>          case MSR_IA32_XSS:
>> -               if (!vmx_xsaves_supported())
>> +               if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) ||
>> +                       !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
>>                          return 1;
> 
> Not complete true.
> 
>>                  /*
>>                   * The only supported bit as of Skylake is bit 8, but
>> @@ -4094,7 +4096,7 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
>>
>>          set_cr4_guest_host_mask(vmx);
>>
>> -       if (vmx_xsaves_supported())
>> +       if (vmx->secondary_exec_control & SECONDARY_EXEC_XSAVES)
>>                  vmcs_write64(XSS_EXIT_BITMAP, VMX_XSS_EXIT_BITMAP);
> 
> This is not true.
> 
> SDM 24.6.20:
> On processors that support the 1-setting of the “enable
> XSAVES/XRSTORS” VM-execution control, the VM-execution control fields
> include a 64-bit XSS-exiting bitmap.
> 
> It depends on whether or not processors support the 1-setting instead
> of “enable XSAVES/XRSTORS” is 1 in VM-exection control field. Anyway,

Yes, whether this field exist or not depends on whether processors 
support the 1-setting.

But if "enable XSAVES/XRSTORS" is clear to 0, XSS_EXIT_BITMAP doesn't 
work. I think in this case, there is no need to set this vmcs field?

> I will send a patch to fix the msr read/write for commit
> 203000993de5(kvm: vmx: add MSR logic for XSAVES), thanks for the
> report.
> 
> Regards,
> Wanpeng Li
>