linux-kernel - Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector

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Message-ID: <87pnnf6dvf.fsf@vitty.brq.redhat.com>
Date:   Fri, 14 Jun 2019 23:44:36 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Prasanna Panchamukhi <panchamukhi@...sta.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Cathy Avery <cavery@...hat.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        Sasha Levin <sashal@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        devel@...uxdriverproject.org, kvm@...r.kernel.org,
        linux-hyperv@...r.kernel.org, x86@...nel.org,
        Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector

Peter Zijlstra <peterz@...radead.org> writes:

>
> I know you probably can't change the HV interface, but I'm thinking its
> rather daft you have to specify a CPU at all for this. The HV can just
> pick one and send the notification there, who cares.

Generally speaking, hypervisor can't know if the CPU is offline (or
e.g. 'isolated') from guest's perspective so I think having an option to
specify affinity for reenlightenment notification is rather a good
thing, not bad.

(Actually, I don't remember if I tried specifying 'HV_ANY' (U32_MAX-1)
here to see what happens. But then I doubt it'll notice the fact that we 
offlined some CPU so we may get a totally unexpected IRQ there).

-- 
Vitaly