linux-kernel - Re: [PATCH v2 3/4] x86/kvm/hyper-v: direct mode for synthetic timers

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Message-ID: <20181210132134.GA5888@rkaganb.sw.ru>
Date:   Mon, 10 Dec 2018 13:21:39 +0000
From:   Roman Kagan <rkagan@...tuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>
Subject: Re: [PATCH v2 3/4] x86/kvm/hyper-v: direct mode for synthetic timers

On Mon, Dec 10, 2018 at 01:54:18PM +0100, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@...tuozzo.com> writes:
> > Just noticed that the patch seems to assume that "direct" timers are
> > allowed to use any vectors including 0-15.  I guess this is incorrect,
> > and instead stimer_set_config should error out on direct mode with a
> > vector less than HV_SYNIC_FIRST_VALID_VECTOR.
> 
> The spec is really vague about this and I'm not sure that this has
> anything to do with HV_SYNIC_FIRST_VALID_VECTOR (as these are actually
> not "synic" vectors, I *think* that SynIC doesn't even need to be
> enabled to make them work).
> 
> I checked and Hyper-V 2016 uses vector '0xff', not sure if it proves
> your point :-)
> 
> Do you envision any issues in KVM if we keep allowing vectors <
> HV_SYNIC_FIRST_VALID_VECTOR?

It's actually lapic that treats vectors 0..15 as illegal.  Nothing
Hyper-V specific here.

Roman.