Re: [PATCH v3 05/34] KVM: x86: hyper-v: Expose support for extended gva ranges for flush hypercalls

Maxim Levitsky
2022-05-11T11:23:41Z

On Thu, 2022-04-14 at 15:19 +0200, Vitaly Kuznetsov wrote:
> Extended GVA ranges support bit seems to indicate whether lower 12
> bits of GVA can be used to specify up to 4095 additional consequent
> GVAs to flush. This is somewhat described in TLFS.
>
> Previously, KVM was handling HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST{,EX}
> requests by flushing the whole VPID so technically, extended GVA
> ranges were already supported. As such requests are handled more
> gently now, advertizing support for extended ranges starts making
> sense to reduce the size of TLB flush requests.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> arch/x86/include/asm/hyperv-tlfs.h | 2 ++
> arch/x86/kvm/hyperv.c | 1 +
> 2 files changed, 3 insertions(+)
>
> diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
> index 0a9407dc0859..5225a85c08c3 100644
> --- a/arch/x86/include/asm/hyperv-tlfs.h
> +++ b/arch/x86/include/asm/hyperv-tlfs.h
> @@ -61,6 +61,8 @@
> #define HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE BIT(10)
> /* Support for debug MSRs available */
> #define HV_FEATURE_DEBUG_MSRS_AVAILABLE BIT(11)
> +/* Support for extended gva ranges for flush hypercalls available */
> +#define HV_FEATURE_EXT_GVA_RANGES_FLUSH BIT(14)
> /*
> * Support for returning hypercall output block via XMM
> * registers is available
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 759e1a16e5c3..1a6f9628cee9 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -2702,6 +2702,7 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
> ent->ebx |= HV_DEBUGGING;
> ent->edx |= HV_X64_GUEST_DEBUGGING_AVAILABLE;
> ent->edx |= HV_FEATURE_DEBUG_MSRS_AVAILABLE;
> + ent->edx |= HV_FEATURE_EXT_GVA_RANGES_FLUSH;
>
> /*
> * Direct Synthetic timers only make sense with in-kernel

I do think that we need to ask Microsoft to document this,
since from the spec (v6.0b) the only mention of this is

"Bit 14: ExtendedGvaRangesForFlushVirtualAddressListAvailable"

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky