linux-kernel - Re: [PATCH 17/17] riscv: add nommu support

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Message-ID: <cbf88fe0-94a6-b559-2b64-c725f236b683@arm.com>
Date:   Tue, 11 Jun 2019 11:32:38 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Christoph Hellwig <hch@....de>, Palmer Dabbelt <palmer@...ive.com>
Cc:     Damien Le Moal <damien.lemoal@....com>,
        linux-riscv@...ts.infradead.org, uclinux-dev@...inux.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 17/17] riscv: add nommu support

On 6/10/19 11:16 PM, Christoph Hellwig wrote:
> Most of the patch is just stubbing out code not needed without page
> tables, but there is an interesting detail in the signals implementation:
> 
>  - The normal RISC-V syscall ABI only implements rt_sigreturn as VDSO
>    entry point, but the ELF VDSO is not supported for nommu Linux.
>    We instead copy the code to call the syscall onto the stack.

On ARM we perform I/D cache synchronization after stack manipulation.

OTOH, ARM port of uClibc provides SA_RESTORER with intention to avoid
manipulation with stack and cache maintenance operations (yet kernel
still performs such manipulation, IIUC, for backward compatibility)

Cheers
Vladimir