linux-kernel - Re: Re: Uncached buffers from CMA DMA heap on some Arm devices?

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Date: Mon, 29 Jan 2024 19:30:29 +0100
From: Pavel Machek <pavel@....cz>
To: Maxime Ripard <mripard@...hat.com>
Cc: Lucas Stach <dev@...xeye.de>,
	kernel list <linux-kernel@...r.kernel.org>,
	Milan Zamazal <mzamazal@...hat.com>, Christoph Hellwig <hch@....de>,
	iommu@...ts.linux.dev, Will Deacon <will@...nel.org>,
	catalin.marinas@....com,
	Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
	Andrey Konovalov <andrey.konovalov.ynk@...il.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	kieran.bingham@...asonboard.com,
	Hans de Goede <hdegoede@...hat.com>
Subject: Re: Re: Uncached buffers from CMA DMA heap on some Arm devices?

Hi!

> > Yeah, and it also does not work :-).
> > 
> > Userspace gets the buffers, and it is not really equipped to work with
> > them. For example, on pinephone, memcpy() crashes on uncached
> > memory. I'm pretty sure user could have some kind of kernel-crashing
> > fun if he passed the uncached memory to futex or something similar.
> 
> Uncached buffers are ubiquitous on arm/arm64 so there must be something
> else going on. And there's nothing to equip for, it's just a memory
> array you can access in any way you want (but very slowly).

Not really. Not on anything modern.

ll/sc will not work, for example, than's on ARM.
https://en.wikipedia.org/wiki/Load-link/store-conditional
Transactional memory will not work, that was on x86. Powerpc has
cacheline clearing instruction.

And that's design, I'm pretty sure there are also numerous CPU errata.

Best regards,
								Pavel
-- 
People of Russia, stop Putin before his war on Ukraine escalates.

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