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

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org,
	"Prasanna Panchamukhi" <panchamukhi@arista.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Borislav Petkov" <bp@alien8.de>,
	"Cathy Avery" <cavery@redhat.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	"Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
	"Mohammed Gamal" <mmorsy@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Roman Kagan" <rkagan@virtuozzo.com>,
	"Sasha Levin" <sashal@kernel.org>,
	"Stephen Hemminger" <sthemmin@microsoft.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	devel@linuxdriverproject.org, kvm@vger.kernel.org,
	linux-hyperv@vger.kernel.org, x86@kernel.org,
	"Dmitry Safonov" <dima@arista.com>
Subject: Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector
Date: Fri, 14 Jun 2019 23:44:36 +0200	[thread overview]
Message-ID: <87pnnf6dvf.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20190614122726.GL3436@hirez.programming.kicks-ass.net>

Peter Zijlstra <peterz@infradead.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