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Date: Mon, 30 Mar 2020 16:22:24 +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 16:04, Will Deacon <will@...nel.org> wrote:
>
> On Mon, Mar 30, 2020 at 03:53:04PM +0200, Ard Biesheuvel wrote:
> > 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?
>
> Sorry, it's not directly related to your patch, just that if we're removing
> options relating to kernel mappings then I'd be quite keen on effectively
> forcing page-granularity on the linear map, as is currently done by default
> thanks to RODATA_FULL_DEFAULT_ENABLED, so that we can nobble cacheable
> aliases for non-coherent streaming DMA mappings by hooking into Christoph's
> series above.
>
Right. I don't remember seeing any complaints about
RODATA_FULL_DEFAULT_ENABLED, but maybe it's too early for that.
> This series just reminded me of it because it's another
> "off-by-default-behaviour-for-block-mappings-probably-because-of-performance-
> but-never-actually-measured" type of thing which really just gets in the
> way.
>
Well, even though I agree that the lack of actual numbers is a bit
disturbing here, I'd hate to penalize all systems even more than they
already are (due to ARCH_KMALLOC_MINALIGN == ARCH_DMA_MINALIGN) by
adding another workaround that is only needed on devices that have
non-coherent masters.
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