linux-kernel - Re: [PATCH 4/4] KVM: hyper-v: Advertise support for fast XMM hypercalls

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Message-ID: <87blap7zha.fsf@vitty.brq.redhat.com>
Date:   Thu, 08 Apr 2021 14:05:53 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Siddharth Chandrasekaran <sidcha@...zon.de>
Cc:     Alexander Graf <graf@...zon.com>,
        Evgeny Iakovlev <eyakovl@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 4/4] KVM: hyper-v: Advertise support for fast XMM
 hypercalls

Siddharth Chandrasekaran <sidcha@...zon.de> writes:

> Now that all extant hypercalls that can use XMM registers (based on
> spec) for input/outputs are patched to support them, we can start
> advertising this feature to guests.
>
> Cc: Alexander Graf <graf@...zon.com>
> Cc: Evgeny Iakovlev <eyakovl@...zon.de>
> Signed-off-by: Siddharth Chandrasekaran <sidcha@...zon.de>
> ---
>  arch/x86/include/asm/hyperv-tlfs.h | 4 ++--
>  arch/x86/kvm/hyperv.c              | 1 +
>  2 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
> index e6cd3fee562b..1f160ef60509 100644
> --- a/arch/x86/include/asm/hyperv-tlfs.h
> +++ b/arch/x86/include/asm/hyperv-tlfs.h
> @@ -49,10 +49,10 @@
>  /* Support for physical CPU dynamic partitioning events is available*/
>  #define HV_X64_CPU_DYNAMIC_PARTITIONING_AVAILABLE	BIT(3)
>  /*
> - * Support for passing hypercall input parameter block via XMM
> + * Support for passing hypercall input and output parameter block via XMM
>   * registers is available
>   */
> -#define HV_X64_HYPERCALL_PARAMS_XMM_AVAILABLE		BIT(4)
> +#define HV_X64_HYPERCALL_PARAMS_XMM_AVAILABLE		BIT(4) | BIT(15)

TLFS 6.0b states that there are two distinct bits for input and output:

CPUID Leaf 0x40000003.EDX:
Bit 4: support for passing hypercall input via XMM registers is available.
Bit 15: support for returning hypercall output via XMM registers is available.

and HV_X64_HYPERCALL_PARAMS_XMM_AVAILABLE is not currently used
anywhere, I'd suggest we just rename 

HV_X64_HYPERCALL_PARAMS_XMM_AVAILABLE to HV_X64_HYPERCALL_XMM_INPUT_AVAILABLE
and add HV_X64_HYPERCALL_XMM_OUTPUT_AVAILABLE (bit 15).

>  /* Support for a virtual guest idle state is available */
>  #define HV_X64_GUEST_IDLE_STATE_AVAILABLE		BIT(5)
>  /* Frequency MSRs available */
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index bf2f86f263f1..dd462c1d641d 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -2254,6 +2254,7 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
>  			ent->ebx |= HV_POST_MESSAGES;
>  			ent->ebx |= HV_SIGNAL_EVENTS;
>  
> +			ent->edx |= HV_X64_HYPERCALL_PARAMS_XMM_AVAILABLE;
>  			ent->edx |= HV_FEATURE_FREQUENCY_MSRS_AVAILABLE;
>  			ent->edx |= HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE;

-- 
Vitaly