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Date: Mon, 08 Jan 2018 22:23:40 +0200
From: Liran Alon <LIRAN.ALON@...CLE.COM>
To: Jim Mattson <jmattson@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>
CC: LKML <linux-kernel@...r.kernel.org>,
kvm list <kvm@...r.kernel.org>, aliguori@...zon.com,
Tom Lendacky <thomas.lendacky@....com>, dwmw@...zon.co.uk,
bp@...en8.de
Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD
down to the guest
On 08/01/18 21:18, Jim Mattson wrote:
> Guest usage of MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD should be
> predicated on guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL).
>
> On Mon, Jan 8, 2018 at 10:08 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>> Direct access to MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD is important
>> for performance. Allow load/store of MSR_IA32_SPEC_CTRL, restore guest
>> IBRS on VM entry and set it to 0 on VM exit (because Linux does not use
>> it yet).
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>> arch/x86/kvm/vmx.c | 32 ++++++++++++++++++++++++++++++++
>> 1 file changed, 32 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 669f5f74857d..d00bcad7336e 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -120,6 +120,8 @@
>> module_param_named(preemption_timer, enable_preemption_timer, bool, S_IRUGO);
>> #endif
>>
>> +static bool __read_mostly have_spec_ctrl;
>> +
>> #define KVM_GUEST_CR0_MASK (X86_CR0_NW | X86_CR0_CD)
>> #define KVM_VM_CR0_ALWAYS_ON_UNRESTRICTED_GUEST (X86_CR0_WP | X86_CR0_NE)
>> #define KVM_VM_CR0_ALWAYS_ON \
>> @@ -609,6 +611,8 @@ struct vcpu_vmx {
>> u64 msr_host_kernel_gs_base;
>> u64 msr_guest_kernel_gs_base;
>> #endif
>> + u64 spec_ctrl;
>> +
>> u32 vm_entry_controls_shadow;
>> u32 vm_exit_controls_shadow;
>> u32 secondary_exec_control;
>> @@ -3361,6 +3365,9 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> case MSR_IA32_TSC:
>> msr_info->data = guest_read_tsc(vcpu);
>> break;
>> + case MSR_IA32_SPEC_CTRL:
>> + msr_info->data = to_vmx(vcpu)->spec_ctrl;
>> + break;
>
> I have:
>
> if (!have_spec_ctrl ||
> (!msr_info->host_initiated &&
> !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
> return 1;
> msr_info->data = to_vmx(vcpu)->msr_ia32_spec_ctrl;
> break;
>
>> case MSR_IA32_SYSENTER_CS:
>> msr_info->data = vmcs_read32(GUEST_SYSENTER_CS);
>> break;
>> @@ -3500,6 +3507,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> case MSR_IA32_TSC:
>> kvm_write_tsc(vcpu, msr_info);
>> break;
>> + case MSR_IA32_SPEC_CTRL:
>> + to_vmx(vcpu)->spec_ctrl = msr_info->data;
>> + break;
>
> I have:
>
> if (!have_spec_ctrl ||
> (!msr_info->host_initiated &&
> !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
> return 1;
> to_vmx(vcpu)->msr_ia32_spec_ctrl = data;
> break;
>
>> case MSR_IA32_CR_PAT:
>> if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {
>> if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
>> @@ -7062,6 +7072,17 @@ static __init int hardware_setup(void)
>> goto out;
>> }
>>
>> + /*
>> + * FIXME: this is only needed until SPEC_CTRL is supported
>> + * by upstream Linux in cpufeatures, then it can be replaced
>> + * with static_cpu_has.
>> + */
>> + have_spec_ctrl = cpu_has_spec_ctrl();
>> + if (have_spec_ctrl)
>> + pr_info("kvm: SPEC_CTRL available\n");
>> + else
>> + pr_info("kvm: SPEC_CTRL not available\n");
>> +
>> if (boot_cpu_has(X86_FEATURE_NX))
>> kvm_enable_efer_bits(EFER_NX);
>>
>> @@ -7131,6 +7152,8 @@ static __init int hardware_setup(void)
>> vmx_disable_intercept_for_msr(MSR_IA32_SYSENTER_CS, false);
>> vmx_disable_intercept_for_msr(MSR_IA32_SYSENTER_ESP, false);
>> vmx_disable_intercept_for_msr(MSR_IA32_SYSENTER_EIP, false);
>> + vmx_disable_intercept_for_msr(MSR_IA32_SPEC_CTRL, false);
>> + vmx_disable_intercept_for_msr(MSR_IA32_PRED_CMD, false);
>
> I have a lot of changes to MSR permission bitmap handling, but these
> intercepts should only be disabled when guest_cpuid_has(vcpu,
> X86_FEATURE_SPEC_CTRL).
>
>> memcpy(vmx_msr_bitmap_legacy_x2apic_apicv,
>> vmx_msr_bitmap_legacy, PAGE_SIZE);
>> @@ -9597,6 +9620,9 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>
>> pt_guest_enter(vmx);
>>
>> + if (have_spec_ctrl && vmx->spec_ctrl != 0)
>> + wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
>> +
>
> Here, I have:
>
> /*
> * If the guest is allowed to write to MSR_IA32_SPEC_CTRL,
> * store it on VM-exit.
> */
> if (guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
> add_autostore_msr(vmx, MSR_IA32_SPEC_CTRL);
> else
> clear_autostore_msr(vmx, MSR_IA32_SPEC_CTRL);
>
> /*
> * If the guest's IA32_SPEC_CTRL MSR doesn't match the host's
> * IA32_SPEC_CTRL MSR, then add the MSR to the atomic switch
> * MSRs, so that the guest value will be loaded on VM-entry
> * and the host value will be loaded on VM-exit.
> */
> if (vmx->msr_ia32_spec_ctrl != spec_ctrl_enabled())
> add_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL,
> vmx->msr_ia32_spec_ctrl,
> spec_ctrl_enabled());
> else
> clear_atomic_switch_msr(vmx, MSR_IA32_SPEC_CTRL);
>
I totally agree with this.
This exactly solves the issue I mentioned before of restoring the guest
value of MSR_IA32_SPEC_CTRL using WRMSR before calling
atomic_switch_perf_msrs() which does an indirect branch.
>> atomic_switch_perf_msrs(vmx);
>>
>> vmx_arm_hv_timer(vcpu);
>> @@ -9707,6 +9733,12 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>> #endif
>> );
>>
>> + if (have_spec_ctrl) {
>> + rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
>> + if (vmx->spec_ctrl)
>> + wrmsrl(MSR_IA32_SPEC_CTRL, 0);
>> + }
>> +
>
> I know the VM-exit MSR load and store lists are probably slower, but
> I'm a little uncomfortable restoring the host's IA32_SPEC_CTRL MSR
> late if the guest has it clear and the host has it set.
Again, I totally agree. This is a better approach for handling this.
>
>> /* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
>> if (vmx->host_debugctlmsr)
>> update_debugctlmsr(vmx->host_debugctlmsr);
>> --
>> 1.8.3.1
>>
>>
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