linux-kernel - Re: [PATCH v2 07/23] KVM: arm64: vgic: Use atomics to count LPIs

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Message-ID: <Zc0HIorNZG9KG5Mg@linux.dev>
Date: Wed, 14 Feb 2024 10:32:02 -0800
From: Oliver Upton <oliver.upton@...ux.dev>
To: Marc Zyngier <maz@...nel.org>
Cc: kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 07/23] KVM: arm64: vgic: Use atomics to count LPIs

Hey,

On Wed, Feb 14, 2024 at 04:47:49PM +0000, Marc Zyngier wrote:
> I'd like to propose an alternative approach here. I've always hated
> this "copy a bunch of INTIDs" thing,

Agree. 

> and the only purpose of this
> silly counter is to dimension the resulting array.

Well, we also use it to trivially print the number of LPIs for a
particular vgic in the debug interface.

> Could we instead rely on an xarray marking a bunch of entries (the
> ones we want to 'copy'), and get the reader to clear these marks once
> done?

I think that'd work. I'm trying to convince myself we don't have bugs
lurking in some of the existing usage of vgic_copy_lpi_list()...

> 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! :)

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.

-- 
Thanks,
Oliver