Messages in this thread | | | Date | Wed, 7 Mar 2018 18:56:11 +0100 | From | Radim Krčmář <> | Subject | Re: [PATCH v2 5/5] x86/kvm: use Enlightened VMCS when running on Hyper-V |
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2018-02-26 18:11+0100, Vitaly Kuznetsov: > Enlightened VMCS is just a structure in memory, the main benefit > besides avoiding somewhat slower VMREAD/VMWRITE is using clean field > mask: we tell the underlying hypervisor which fields were modified > since VMEXIT so there's no need to inspect them all. > > Tight CPUID loop test shows significant speedup: > Before: 20766 cycles > After: 8912 cycles > > Static key is being used to avoid performance penalty for non-Hyper-V > deployments. Tests show we add around 3 (three) CPU cycles on each > VMEXIT (1077.5 cycles before, 1080.7 cycles after for the same CPUID > loop on bare metal). We can probably avoid one test/jmp in vmx_vcpu_run() > but I don't see a clean way to use static key in assembly.
Patching the correct instruction should be simpler than replicating static_branch (because we have to support assemblers without ASM_GOTO), but I'd care about that later, if ever.
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com> > --- > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > @@ -999,6 +1000,442 @@ static const u32 vmx_msr_index[] = { > +/* > + * Enlightened VMCSv1 doesn't support these:
I take it that the code assumes that the hypervisor will never enable these features if we have enlightened VMCS. I think it would be best to disable those features when turning on evmcs.
> + * POSTED_INTR_NV = 0x00000002, > + * GUEST_INTR_STATUS = 0x00000810, > + * APIC_ACCESS_ADDR = 0x00002014, > + * POSTED_INTR_DESC_ADDR = 0x00002016, > + * EOI_EXIT_BITMAP0 = 0x0000201c, > + * EOI_EXIT_BITMAP1 = 0x0000201e, > + * EOI_EXIT_BITMAP2 = 0x00002020, > + * EOI_EXIT_BITMAP3 = 0x00002022,
enable_apicv, flexpriority_enabled
> + * GUEST_PML_INDEX = 0x00000812, > + * PML_ADDRESS = 0x0000200e,
enable_pml
> + * VM_FUNCTION_CONTROL = 0x00002018, > + * EPTP_LIST_ADDRESS = 0x00002024,
(only vm controls)
> + * VMREAD_BITMAP = 0x00002026, > + * VMWRITE_BITMAP = 0x00002028,
enable_shadow_vmcs
> + * TSC_MULTIPLIER = 0x00002032,
(only vm controls)
> + * GUEST_IA32_PERF_GLOBAL_CTRL = 0x00002808, > + * GUEST_IA32_RTIT_CTL = 0x00002814, > + * HOST_IA32_PERF_GLOBAL_CTRL = 0x00002c04,
(only vm controls)
> + * PLE_GAP = 0x00004020, > + * PLE_WINDOW = 0x00004022,
ple_gap
> + * VMX_PREEMPTION_TIMER_VALUE = 0x0000482E,
enable_preemption_timer
> + */ > +}; > + > +static inline u16 get_evmcs_offset(unsigned long field) > +{ > + unsigned int index = ROL16(field, 6); > + > + if (index >= ARRAY_SIZE(vmcs_field_to_evmcs_1)) > + return 0;
Please add a warning when trying to use an EVMCS that doesn't exist, just like the VMREAD/WRITE does -- it's an internal KVM error.
> + > + return (u16)vmcs_field_to_evmcs_1[index]; > +} > + > @@ -3828,7 +4302,12 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf) > vmcs_conf->size = vmx_msr_high & 0x1fff; > vmcs_conf->order = get_order(vmcs_conf->size); > vmcs_conf->basic_cap = vmx_msr_high & ~0x1fff; > - vmcs_conf->revision_id = vmx_msr_low; > + > + /* KVM supports Enlightened VMCS v1 only */ > + if (static_branch_unlikely(&enable_evmcs)) > + vmcs_conf->revision_id = 1;
I think we have to put the bottom bits from ms_hyperv.nested_features here. 16.5.1 Enlightened VMCS Versioning:
Each enlightened VMCS structure contains a version field, which is reported by the L0 hypervisor (see 2.4.11)
> + else > + vmcs_conf->revision_id = vmx_msr_low; > > vmcs_conf->pin_based_exec_ctrl = _pin_based_exec_control; > vmcs_conf->cpu_based_exec_ctrl = _cpu_based_exec_control; > @@ -12414,7 +12908,35 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = { > > static int __init vmx_init(void) > { > - int r = kvm_init(&vmx_x86_ops, sizeof(struct vcpu_vmx), > + int r; > + > +#if IS_ENABLED(CONFIG_HYPERV) > + /* > + * Enlightened VMCS usage should be recommended and the host needs > + * to support eVMCS v1 or above. We can also disable eVMCS support > + * with module parameter. > + */ > + if (enlightened_vmcs && > + ms_hyperv.hints & HV_X64_ENLIGHTENED_VMCS_RECOMMENDED && > + (ms_hyperv.nested_features & 0xff) >= 1) {
Please add macros like HV_X64_ENLIGHTENED_VMCS_VERSION and KVM_EVMCS_VERSION for those numbers. I think we should not proceed with version other than 1 for now -- there is no mention of backward compatibility in the spec.
Thanks.
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