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Date: Mon, 29 Jan 2024 21:04:50 -0500
From: "Stefan O'Rear" <sorear@fastmail.com>
To: "Charlie Jenkins" <charlie@rivosinc.com>,
 "Alexandre Ghiti" <alexghiti@rivosinc.com>,
 "Paul Walmsley" <paul.walmsley@sifive.com>,
 "Palmer Dabbelt" <palmer@dabbelt.com>, "Albert Ou" <aou@eecs.berkeley.edu>,
 "Shuah Khan" <shuah@kernel.org>, "Jonathan Corbet" <corbet@lwn.net>,
 "Yangyu Chen" <cyy@cyyself.name>
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
 linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH 0/3] riscv: mm: Use hint address in mmap if available
Content-Type: text/plain

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.

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?

-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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