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

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Date:   Thu, 8 Apr 2021 15:44:46 +0000
From:   Wei Liu <wei.liu@...nel.org>
To:     Siddharth Chandrasekaran <sidcha@...zon.de>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        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 04:20:54PM +0200, Siddharth Chandrasekaran wrote:
> 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.
> 

They are two separate things in TLFS. Please use two macros here.

Wei.