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

[Guenter Roeck]

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@xxxxxxxxxx>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>

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).

Reverting the patch fixes the problem.

Guenter

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# bad: [44832877ea4c186460f42e48b1da98a08a29ffb5] ARM: 8942/1: Revert "8857/1: efi: enable CP15 DMB instructions before cleaning the cache"
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# first bad commit: [bcb704e551fc4e5034b9fa319a3ef49d30cb84d9] ARM: 8936/1: decompressor: avoid CP15 barrier instructions in v7 cache setup code