Received-SPF: pass (google.com: domain of linux-kernel+bounces-66570-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Date: Thu, 15 Feb 2024 09:44:06 +0000
Message-ID: <86sf1u3vvd.wl-maz@kernel.org>
From: Marc Zyngier <maz@kernel.org>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: kvmarm@lists.linux.dev,
	kvm@vger.kernel.org,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 07/23] KVM: arm64: vgic: Use atomics to count LPIs
In-Reply-To: <Zc1GMFjvy_f1KsXr@linux.dev>
References: <20240213093250.3960069-1-oliver.upton@linux.dev>
	<20240213093250.3960069-8-oliver.upton@linux.dev>
	<861q9f56x6.wl-maz@kernel.org>
	<Zc0HIorNZG9KG5Mg@linux.dev>
	<86wmr64xyo.wl-maz@kernel.org>
	<Zc1GMFjvy_f1KsXr@linux.dev>
User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue)
 FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/29.1
 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO)
Precedence: bulk
MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue")
Content-Type: text/plain; charset=US-ASCII

On Wed, 14 Feb 2024 23:01:04 +0000,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> On Wed, Feb 14, 2024 at 08:01:19PM +0000, Marc Zyngier wrote:
> > > > Of course, we only have 3 marks, so that's a bit restrictive from a
> > > > concurrency perspective, but since most callers hold a lock, it should
> > > > be OK.
> > > 
> > > They all hold *a* lock, but maybe not the same one! :)
> > 
> > Indeed. But as long as there isn't more than 3 locks (and that the
> > xarray is OK being concurrently updated with marks), we're good!
> 
> Oh, you mean to give each existing caller their own mark?

Well, each caller "class". Where "class" means "holding look
'foo'". Same lock, same mark. With a maximum of 3 (and I think we can
get away with 2).

> > > Maybe we should serialize the use of markers on the LPI list on the
> > > config_lock. A slight misuse, but we need a mutex since we're poking at
> > > guest memory. Then we can go through the whole N-dimensional locking
> > > puzzle and convince ourselves it is still correct.
> > 
> > Maybe. This thing is already seeing so many abuses that one more may
> > not matter much. Need to see how it fits in the whole hierarchy of
> > GIC-related locks...
> 
> It doesn't work. We have it that the config_lock needs to be taken
> outside the its_lock.
> 
> Too many damn locks!

Well, the joys of emulating highly complex HW with a braindead
programming interface. I'd explore the above suggestion to avoid
introducing a new lock, if at all possible.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.