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SubjectRe: [PATCH v6 12/20] kvm/vmx: Emulate MSR TEST_CTL
On Wed, Apr 03, 2019 at 02:21:58PM -0700, Fenghua Yu wrote:
> From: Xiaoyao Li <xiaoyao.li@linux.intel.com>
>
> 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 <xiaoyao.li@linux.intel.com>
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> Acked-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> 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
>

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