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Date: Mon, 30 Mar 2020 15:53:04 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Will Deacon <will@...nel.org>
Cc: Linux ARM <linux-arm-kernel@...ts.infradead.org>,
kernel-hardening@...ts.openwall.com,
Catalin Marinas <catalin.marinas@....com>, Mark Rutland <mark.rutland@....com>
Subject: Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature
On Mon, 30 Mar 2020 at 15:51, Will Deacon <will@...nel.org> wrote:
>
> 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
>
I'm not sure I follow - is this about mapping parts of the static
kernel Image for non-coherent DMA?
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