Subject: Re: [PATCH v6 12/20] kvm/vmx: Emulate MSR TEST_CTL

From: Sean Christopherson <[email-masked]>
Date: Thu, 4 Apr 2019 07:44:02 -0700

On Wed, Apr 03, 2019 at 02:21:58PM -0700, Fenghua Yu wrote:
> From: Xiaoyao Li <[email-masked]>
> 
> A control bit (bit 29) in TEST_CTL MSR 0x33 will be introduced in
> future x86 processors. When bit 29 is set, the processor causes #AC
> exception for split locked accesses at all CPL.
> 
> Please check the latest Intel 64 and IA-32 Architectures Software
> Developer's Manual for more detailed information on the MSR and
> the split lock bit.
> 
> This patch emulate MSR TEST_CTL with vmx->msr_test_ctl and does the
> following:
> 1. As MSR TEST_CTL of guest is emulated, enable the related bits
> in CORE_CAPABILITY to corretly report this feature to guest.

s/corretly/correctly

> 
> 2. Differentiate MSR TEST_CTL between host and guest.
> 
> Signed-off-by: Xiaoyao Li <[email-masked]>
> Signed-off-by: Fenghua Yu <[email-masked]>
> Acked-by: Paolo Bonzini <[email-masked]>
> ---
>  arch/x86/kvm/vmx/vmx.c | 35 +++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/vmx/vmx.h |  1 +
>  arch/x86/kvm/x86.c     | 17 ++++++++++++++++-
>  3 files changed, 52 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ab432a930ae8..309ccf593f0d 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1663,6 +1663,12 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	u32 index;
>  
>  	switch (msr_info->index) {
> +	case MSR_TEST_CTL:
> +		if (!msr_info->host_initiated &&
> +		    !(vcpu->arch.core_capability & CORE_CAP_SPLIT_LOCK_DETECT))
> +			return 1;
> +		msr_info->data = vmx->msr_test_ctl;
> +		break;
>  #ifdef CONFIG_X86_64
>  	case MSR_FS_BASE:
>  		msr_info->data = vmcs_readl(GUEST_FS_BASE);
> @@ -1797,6 +1803,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	u32 index;
>  
>  	switch (msr_index) {
> +	case MSR_TEST_CTL:
> +		if (!(vcpu->arch.core_capability & CORE_CAP_SPLIT_LOCK_DETECT))
> +			return 1;
> +
> +		if (data & ~TEST_CTL_ENABLE_SPLIT_LOCK_DETECT)
> +			return 1;
> +		vmx->msr_test_ctl = data;
> +		break;
>  	case MSR_EFER:
>  		ret = kvm_set_msr_common(vcpu, msr_info);
>  		break;
> @@ -4077,6 +4091,9 @@ static void vmx_vcpu_setup(struct vcpu_vmx *vmx)
>  		++vmx->nmsrs;
>  	}
>  
> +	/* disable AC split lock by default */
> +	vmx->msr_test_ctl = 0;
> +
>  	vm_exit_controls_init(vmx, vmx_vmexit_ctrl());
>  
>  	/* 22.2.1, 20.8.1 */
> @@ -4114,6 +4131,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  
>  	vmx->rmode.vm86_active = 0;
>  	vmx->spec_ctrl = 0;
> +	vmx->msr_test_ctl = 0;
>  
>  	vcpu->arch.microcode_version = 0x100000000ULL;
>  	vmx->vcpu.arch.regs[VCPU_REGS_RDX] = get_rdx_init_val();
> @@ -6313,6 +6331,21 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
>  					msrs[i].host, false);
>  }
>  
> +static void atomic_switch_msr_test_ctl(struct vcpu_vmx *vmx)
> +{
> +	u64 host_msr_test_ctl;
> +
> +	/* if TEST_CTL MSR doesn't exist on the hardware, do nothing */
> +	if (rdmsrl_safe(MSR_TEST_CTL, &host_msr_test_ctl))
> +		return;

This adds a RDMSR on every VM-Enter, and a fault on CPUs that don't
support MSR_TEST_CTL.  Ideally the kernel would cache MSR_TEST_CTL and
expose a helper that returns a boolean to indicate the existence of the
MSRs along with the current value.  Racing with split_lock_detect_store()
is ok since this code runs with interrupts disabled, i.e. will block
split_lock_detect_store() until after VM-Exit.

Paolo, can you weigh in with your thoughts?  I'm surprised you acked
this patch given your earlier comment:

https://patchwork.kernel.org/patch/10413779/#21892723

> +
> +	if (host_msr_test_ctl == vmx->msr_test_ctl)
> +		clear_atomic_switch_msr(vmx, MSR_TEST_CTL);
> +	else
> +		add_atomic_switch_msr(vmx, MSR_TEST_CTL, vmx->msr_test_ctl,
> +				      host_msr_test_ctl, false);
> +}
> +
>  static void vmx_arm_hv_timer(struct vcpu_vmx *vmx, u32 val)
>  {
>  	vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, val);
> @@ -6419,6 +6452,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  
>  	atomic_switch_perf_msrs(vmx);
>  
> +	atomic_switch_msr_test_ctl(vmx);
> +
>  	vmx_update_hv_timer(vcpu);
>  
>  	/*
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index a1e00d0a2482..6091a8b9de74 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -190,6 +190,7 @@ struct vcpu_vmx {
>  	u64		      msr_guest_kernel_gs_base;
>  #endif
>  
> +	u64		      msr_test_ctl;
>  	u64		      spec_ctrl;
>  
>  	u32 vm_entry_controls_shadow;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4459115eb0ec..e93c2f620cdb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1229,7 +1229,22 @@ EXPORT_SYMBOL_GPL(kvm_get_arch_capabilities);
>  
>  u64 kvm_get_core_capability(void)
>  {
> -	return 0;
> +	u64 data;
> +
> +	rdmsrl_safe(MSR_IA32_CORE_CAPABILITY, &data);
> +
> +	/* mask non-virtualizable functions */
> +	data &= CORE_CAP_SPLIT_LOCK_DETECT;
> +
> +	/*
> +	 * There will be a list of FMS values that have split lock detection
> +	 * but lack the CORE CAPABILITY MSR. In this case, set
> +	 * CORE_CAP_SPLIT_LOCK_DETECT since we emulate MSR CORE_CAPABILITY.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
> +		data |= CORE_CAP_SPLIT_LOCK_DETECT;
> +
> +	return data;
>  }
>  EXPORT_SYMBOL_GPL(kvm_get_core_capability);
>  
> -- 
> 2.19.1
>