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

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Message-ID: <86wmr64xyo.wl-maz@kernel.org>
Date: Wed, 14 Feb 2024 20:01:19 +0000
From: Marc Zyngier <maz@...nel.org>
To: Oliver Upton <oliver.upton@...ux.dev>
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

On Wed, 14 Feb 2024 18:32:02 +0000,
Oliver Upton <oliver.upton@...ux.dev> wrote:
> 
> 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.

I think we can get survive this... ;-)

> 
> > 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! :)

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!

> 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...

Thanks,

	M.

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