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

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Message-ID: <20210408142053.GA10636@u366d62d47e3651.ant.amazon.com>
Date:   Thu, 8 Apr 2021 16:20:54 +0200
From:   Siddharth Chandrasekaran <sidcha@...zon.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
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

On Thu, Apr 08, 2021 at 02:05:53PM +0200, Vitaly Kuznetsov wrote:
> 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).

That is how I had it initially; but then noticed that we would never
need to use either of them separately. So it seemed like a reasonable
abstraction to put them together.

~ Sid.



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