Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves_supported

From: Wanpeng Li
Date: Thu Jun 20 2019 - 02:44:16 EST


Hi,
On Thu, 20 Jun 2019 at 13:06, Tao Xu <tao3.xu@xxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxxxxxxxx>
> Signed-off-by: Tao Xu <tao3.xu@xxxxxxxxx>
> ---
> 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 âenable
XSAVES/XRSTORSâ 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 âenable XSAVES/XRSTORSâ is 1 in VM-exection control field. Anyway,
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