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From: Sean Christopherson <seanjc@google.com>
To: Yang Zhong <yang.zhong@intel.com>
Cc: x86@kernel.org, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-kselftest@vger.kernel.org, tglx@linutronix.de,
	mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
	pbonzini@redhat.com, corbet@lwn.net, shuah@kernel.org,
	jun.nakajima@intel.com, kevin.tian@intel.com,
	jing2.liu@linux.intel.com, jing2.liu@intel.com,
	guang.zeng@intel.com, wei.w.wang@intel.com
Subject: Re: [PATCH v4 10/21] kvm: x86: Add emulation for IA32_XFD
Date: Tue, 4 Jan 2022 19:32:30 +0000	[thread overview]
Message-ID: <YdSgzsiBft/NgxdF@google.com> (raw)
In-Reply-To: <20211229131328.12283-11-yang.zhong@intel.com>

On Wed, Dec 29, 2021, Yang Zhong wrote:
> From: Jing Liu <jing2.liu@intel.com>
> 
> Intel's eXtended Feature Disable (XFD) feature allows the software
> to dynamically adjust fpstate buffer size for XSAVE features which
> have large state.
> 
> Because fpstate has been expanded for all possible dynamic xstates
> at KVM_SET_CPUID2, emulation of the IA32_XFD MSR is straightforward.
> For write just call fpu_update_guest_xfd() to update the guest fpu
> container once all the sanity checks are passed. For read then
> return the cached value in the container.
> 
> Signed-off-by: Zeng Guang <guang.zeng@intel.com>
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> Signed-off-by: Jing Liu <jing2.liu@intel.com>
> Signed-off-by: Yang Zhong <yang.zhong@intel.com>
> ---
>  arch/x86/kvm/x86.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e50e97ac4408..36677b754ac9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1359,6 +1359,7 @@ static const u32 msrs_to_save_all[] = {
>  	MSR_F15H_PERF_CTL3, MSR_F15H_PERF_CTL4, MSR_F15H_PERF_CTL5,
>  	MSR_F15H_PERF_CTR0, MSR_F15H_PERF_CTR1, MSR_F15H_PERF_CTR2,
>  	MSR_F15H_PERF_CTR3, MSR_F15H_PERF_CTR4, MSR_F15H_PERF_CTR5,
> +	MSR_IA32_XFD,
>  };
>  
>  static u32 msrs_to_save[ARRAY_SIZE(msrs_to_save_all)];
> @@ -3669,6 +3670,19 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  			return 1;
>  		vcpu->arch.msr_misc_features_enables = data;
>  		break;
> +#ifdef CONFIG_X86_64
> +	case MSR_IA32_XFD:
> +		if (!msr_info->host_initiated &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_XFD))
> +			return 1;
> +
> +		if (data & ~(XFEATURE_MASK_USER_DYNAMIC &
> +			     vcpu->arch.guest_supported_xcr0))
> +			return 1;
> +
> +		fpu_update_guest_xfd(&vcpu->arch.guest_fpu, data);
> +		break;
> +#endif
>  	default:
>  		if (kvm_pmu_is_valid_msr(vcpu, msr))
>  			return kvm_pmu_set_msr(vcpu, msr_info);
> @@ -3989,6 +4003,15 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	case MSR_K7_HWCR:
>  		msr_info->data = vcpu->arch.msr_hwcr;
>  		break;
> +#ifdef CONFIG_X86_64
> +	case MSR_IA32_XFD:
> +		if (!msr_info->host_initiated &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_XFD))
> +			return 1;
> +
> +		msr_info->data = vcpu->arch.guest_fpu.fpstate->xfd;
> +		break;
> +#endif
>  	default:
>  		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
>  			return kvm_pmu_get_msr(vcpu, msr_info);
> @@ -6422,6 +6445,10 @@ static void kvm_init_msr_list(void)
>  			    min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
>  				continue;
>  			break;
> +		case MSR_IA32_XFD:
> +			if (!kvm_cpu_cap_has(X86_FEATURE_XFD))
> +				continue;

I suspect the 32-bit host support is wrong.  The kernel's handle_xfd_event()
checks for 64-bit support in addition to the CPU feature itself, which implies
that the feature can be reported in boot_cpu_data for 32-bit kernels.

  static bool handle_xfd_event(struct pt_regs *regs)
  {
	u64 xfd_err;
	int err;

	if (!IS_ENABLED(CONFIG_X86_64) || !cpu_feature_enabled(X86_FEATURE_XFD))
		return false;

	...
  }

In this specific case, that means KVM will tell userspace it needs to mgirate
MSR_IA32_XFD, and then reject attempts to read/write the MSR.

If 32-bit host kernels do not explicitly suppress X86_FEATURE_XFD, then KVM needs
to do:

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 556555537a18..156ce332d55b 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -455,9 +455,11 @@ void kvm_set_cpu_caps(void)
 #ifdef CONFIG_X86_64
        unsigned int f_gbpages = F(GBPAGES);
        unsigned int f_lm = F(LM);
+       unsigned int f_xfd = F(XFD);
 #else
        unsigned int f_gbpages = 0;
        unsigned int f_lm = 0;
+       unsigned int f_xfd = 0;
 #endif
        memset(kvm_cpu_caps, 0, sizeof(kvm_cpu_caps));

@@ -545,7 +547,7 @@ void kvm_set_cpu_caps(void)
        );

        kvm_cpu_cap_mask(CPUID_D_1_EAX,
-               F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | F(XFD)
+               F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | F(XSAVES) | f_xfd
        );

        kvm_cpu_cap_init_scattered(CPUID_12_EAX,

> +			break;
>  		default:
>  			break;
>  		}

  reply	other threads:[~2022-01-04 19:32 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-12-29 13:13 [PATCH v4 00/21] AMX Support in KVM Yang Zhong
2021-12-29 13:13 ` [PATCH v4 01/21] x86/fpu: Extend fpu_xstate_prctl() with guest permissions Yang Zhong
2021-12-29 13:13 ` [PATCH v4 02/21] x86/fpu: Prepare guest FPU for dynamically enabled FPU features Yang Zhong
2021-12-29 13:13 ` [PATCH v4 03/21] kvm: x86: Fix xstate_required_size() to follow XSTATE alignment rule Yang Zhong
2022-01-04 19:54   ` Sean Christopherson
2021-12-29 13:13 ` [PATCH v4 04/21] kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID Yang Zhong
2021-12-29 13:13 ` [PATCH v4 05/21] x86/fpu: Make XFD initialization in __fpstate_reset() a function argument Yang Zhong
2021-12-29 13:13 ` [PATCH v4 06/21] x86/fpu: Add guest support to xfd_enable_feature() Yang Zhong
2021-12-29 13:13 ` [PATCH v4 07/21] x86/fpu: Provide fpu_update_guest_perm_features() for guest Yang Zhong
2021-12-29 13:13 ` [PATCH v4 08/21] kvm: x86: Check and enable permitted dynamic xfeatures at KVM_SET_CPUID2 Yang Zhong
2021-12-29 16:55   ` Sean Christopherson
2021-12-30  2:28     ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 09/21] x86/fpu: Provide fpu_update_guest_xfd() for IA32_XFD emulation Yang Zhong
2021-12-29 13:13 ` [PATCH v4 10/21] kvm: x86: Add emulation for IA32_XFD Yang Zhong
2022-01-04 19:32   ` Sean Christopherson [this message]
2022-01-05  0:22     ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 11/21] x86/fpu: Prepare xfd_err in struct fpu_guest Yang Zhong
2021-12-29 13:13 ` [PATCH v4 12/21] kvm: x86: Intercept #NM for saving IA32_XFD_ERR Yang Zhong
2022-01-04 20:01   ` Sean Christopherson
2022-01-05  0:27     ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 13/21] kvm: x86: Emulate IA32_XFD_ERR for guest Yang Zhong
2021-12-29 13:13 ` [PATCH v4 14/21] kvm: x86: Disable RDMSR interception of IA32_XFD_ERR Yang Zhong
2022-01-04 19:34   ` Sean Christopherson
2022-01-05  0:23     ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 15/21] kvm: x86: Add XCR0 support for Intel AMX Yang Zhong
2021-12-29 13:13 ` [PATCH v4 16/21] kvm: x86: Add CPUID " Yang Zhong
2021-12-29 13:13 ` [PATCH v4 17/21] x86/fpu: Add uabi_size to guest_fpu Yang Zhong
2021-12-29 13:13 ` [PATCH v4 18/21] kvm: x86: Add support for getting/setting expanded xstate buffer Yang Zhong
2022-01-04 19:46   ` Sean Christopherson
2022-01-04 20:45     ` Paolo Bonzini
2022-01-05  4:03       ` Tian, Kevin
2021-12-29 13:13 ` [PATCH v4 19/21] kvm: selftests: Add support for KVM_CAP_XSAVE2 Yang Zhong
2021-12-29 13:13 ` [PATCH v4 20/21] x86/fpu: Provide fpu_sync_guest_vmexit_xfd_state() Yang Zhong
2021-12-29 13:13 ` [PATCH v4 21/21] kvm: x86: Disable interception for IA32_XFD on demand Yang Zhong
2021-12-30  8:46 ` [PATCH v4 00/21] AMX Support in KVM Tian, Kevin
2022-01-04 18:36 ` Paolo Bonzini
2022-01-04 18:54   ` Sean Christopherson
2022-01-05  0:10     ` Tian, Kevin

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