Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature

From: Will Deacon <will@kernel.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
	kernel-hardening@lists.openwall.com,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>
Subject: Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature
Date: Mon, 30 Mar 2020 15:04:42 +0100	[thread overview]
Message-ID: <20200330140441.GE10633@willie-the-truck> (raw)
In-Reply-To: <CAMj1kXEZARZ1FYZFt4CZ33b-A64zj1JswR0OAHw-eZdzkxiEOQ@mail.gmail.com>

On Mon, Mar 30, 2020 at 03:53:04PM +0200, Ard Biesheuvel wrote:
> On Mon, 30 Mar 2020 at 15:51, Will Deacon <will@kernel.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@kernel.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@kernel.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.

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.

Will