linux-kernel - Re: [patches] RE: [PATCH 3/4] RISC-V: Flush I$ when making a dirty page executable

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Date:   Wed, 22 Nov 2017 09:38:16 -0800 (PST)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     David.Laight@...LAB.COM
CC:     linux-kernel@...r.kernel.org, patches@...ups.riscv.org,
        Andrew Waterman <andrew@...ive.com>
Subject:     Re: [patches] RE: [PATCH 3/4] RISC-V: Flush I$ when making a dirty page executable

On Tue, 21 Nov 2017 08:57:07 PST (-0800), David.Laight@...LAB.COM wrote:
> From: Palmer Dabbelt
>> Sent: 20 November 2017 18:58
>>
>> The RISC-V ISA allows for instruction caches that are not coherent WRT
>> stores, even on a single hart.  As a result, we need to explicitly flush
>> the instruction cache whenever marking a dirty page as executable in
>> order to preserve the correct system behavior.
>
> Isn't the I-flush only needed if there has been an unmap since the
> previous I-flush?
> Since code is rarely unmapped (exec and driver unload come to mind)
> the I-flush won't be needed very often.

There's nothing in the RISC-V ISA that prevents the instruction cache from 
caching read-only (or even unmapped!) pages.  Instructions fetched this manner 
could never commit, but they could fill up the icache with garbage.  I believe 
that means we need to flush on dirty->execute, but if I'm wrong I'm happy to 
change it.