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SubjectRe: [PATCH v2 5/5] x86/kvm: use Enlightened VMCS when running on Hyper-V
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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