Re: [PATCH] ARM: 8936/1: decompressor: avoid CP15 barrier instructions in v7 cache setup code - Russell King

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Guenter Roeck <linux@roeck-us.net>
Cc: Marc Zyngier <maz@kernel.org>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: Re: [PATCH] ARM: 8936/1: decompressor: avoid CP15 barrier instructions in v7 cache setup code
Date: Sat, 25 Jan 2020 18:06:13 +0000	[thread overview]
Message-ID: <20200125180613.GR25745@shell.armlinux.org.uk> (raw)
In-Reply-To: <20200125173950.GA19126@roeck-us.net>

On Sat, Jan 25, 2020 at 09:39:50AM -0800, Guenter Roeck wrote:
> Hi,
> 
> On Fri, Nov 08, 2019 at 01:44:32PM +0100, Ard Biesheuvel wrote:
> > Commit e17b1af96b2afc38e684aa2f1033387e2ed10029
> > 
> >   "ARM: 8857/1: efi: enable CP15 DMB instructions before cleaning the cache"
> > 
> > added some explicit handling of the CP15BEN bit in the SCTLR system
> > register, to ensure that CP15 barrier instructions are enabled, even
> > if we enter the decompressor via the EFI stub.
> > 
> > However, as it turns out, there are other ways in which we may end up
> > using CP15 barrier instructions without them being enabled. I.e., when
> > the decompressor startup code skips the cache_on() initially, we end
> > up calling cache_clean_flush() with the caches and MMU off, in which
> > case the CP15BEN bit in SCTLR may not be programmed either. And in
> > fact, cache_on() itself issues CP15 barrier instructions before actually
> > enabling them by programming the new SCTLR value (and issuing an ISB)
> > 
> > Since all these routines are specific to v7, let's clean this up by
> > using the ordinary v7 barrier instructions in the v7 specific cache
> > handling routines, so that we never rely on the CP15 ones. This also
> > avoids the issue where a barrier is required between programming SCTLR
> > and using the CP15 barrier instructions, which would result in two
> > different kinds of barriers being used in the same function.
> > 
> > Acked-by: Marc Zyngier <maz@kernel.org>
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
> 
> This patch causes all qemu emulations for ARM1176 to fail hard (stall with
> no console output even with earlycon enabled). This affects witherspoon-bmc,
> ast2500-evb, romulus-bmc, and swift-bmc. It does not affect emulations
> for other CPU types, even with the same kernel configuration (such as
> ast2600-evb).

Hmm, looks like we're going to have to drop 8936/1, 8941/1 and 8942/1
in that case.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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