Re: [PATCH 0/3] riscv: mm: Use hint address in mmap if available

[Charlie Jenkins <charlie@rivosinc.com>]

On Mon, Jan 29, 2024 at 09:04:50PM -0500, Stefan O'Rear wrote:
> On Mon, Jan 29, 2024, at 7:36 PM, Charlie Jenkins wrote:
> > On riscv, mmap currently returns an address from the largest address
> > space that can fit entirely inside of the hint address. This makes it
> > such that the hint address is almost never returned. This patch raises
> > the mappable area up to and including the hint address. This allows mmap
> > to often return the hint address, which allows a performance improvement
> > over searching for a valid address as well as making the behavior more
> > similar to other architectures.
> 
> This means that if an application or library opts in to Sv48 support by
> passing a nonzero hint, it will lose the benefits of ASLR.

sv48 is default. However your statement stands for opting into sv57.
If they always pass the same hint address, only the first address will
be deterministic though, correct?

> 
> Allowing applications to opt in to a VA space smaller than the
> architectural minimum seems like an independently useful feature.
> Is there a reason to only add it for riscv64?
> 

If there is interest, it can be added to other architectures as well.

- Charlie

> -s
> 
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> > Charlie Jenkins (3):
> >       riscv: mm: Use hint address in mmap if available
> >       selftests: riscv: Generalize mm selftests
> >       docs: riscv: Define behavior of mmap
> >
> >  Documentation/arch/riscv/vm-layout.rst           | 16 ++--
> >  arch/riscv/include/asm/processor.h               | 21 ++----
> >  tools/testing/selftests/riscv/mm/mmap_bottomup.c | 20 +----
> >  tools/testing/selftests/riscv/mm/mmap_default.c  | 20 +----
> >  tools/testing/selftests/riscv/mm/mmap_test.h     | 93 +++++++++++++-----------
> >  5 files changed, 66 insertions(+), 104 deletions(-)
> > ---
> > base-commit: 556e2d17cae620d549c5474b1ece053430cd50bc
> > change-id: 20240119-use_mmap_hint_address-f9f4b1b6f5f1
> > -- 
> > - Charlie
> >
> >
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