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Date: Mon, 30 Mar 2020 14:51:21 +0100
From: Will Deacon <will@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org,
kernel-hardening@...ts.openwall.com, catalin.marinas@....com,
mark.rutland@....com
Subject: Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature
On Sun, Mar 29, 2020 at 04:12:58PM +0200, Ard Biesheuvel wrote:
> When CONFIG_DEBUG_ALIGN_RODATA is enabled, kernel segments mapped with
> different permissions (r-x for .text, r-- for .rodata, rw- for .data,
> etc) are rounded up to 2 MiB so they can be mapped more efficiently.
> In particular, it permits the segments to be mapped using level 2
> block entries when using 4k pages, which is expected to result in less
> TLB pressure.
>
> However, the mappings for the bulk of the kernel will use level 2
> entries anyway, and the misaligned fringes are organized such that they
> can take advantage of the contiguous bit, and use far fewer level 3
> entries than would be needed otherwise.
>
> This makes the value of this feature dubious at best, and since it is not
> enabled in defconfig or in the distro configs, it does not appear to be
> in wide use either. So let's just remove it.
>
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
> arch/arm64/Kconfig.debug | 13 -------------
> arch/arm64/include/asm/memory.h | 12 +-----------
> drivers/firmware/efi/libstub/arm64-stub.c | 8 +++-----
> 3 files changed, 4 insertions(+), 29 deletions(-)
Acked-by: Will Deacon <will@...nel.org>
But I would really like to go a step further and rip out the block mapping
support altogether so that we can fix non-coherent DMA aliases:
https://lore.kernel.org/lkml/20200224194446.690816-1-hch@lst.de
Will
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