On 2020-03-30 2:53 pm, Ard Biesheuvel wrote:
On Mon, 30 Mar 2020 at 15:51, Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxx>
---
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@xxxxxxxxxx>
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@xxxxxx
I'm not sure I follow - is this about mapping parts of the static
kernel Image for non-coherent DMA?
Yikes, I hope not!
The concern there is about block entries in the linear map; I'd assume
kernel text/data means not-linear-map, and is thus a different kettle of
fish anyway.
Robin.
