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Re: [PATCH v4 01/10] target/riscv: turn write_misa() into an official no

From: Bin Meng
Subject: Re: [PATCH v4 01/10] target/riscv: turn write_misa() into an official no-op
Date: Thu, 16 Feb 2023 21:26:50 +0800

On Thu, Feb 16, 2023 at 9:06 PM Daniel Henrique Barboza
<dbarboza@ventanamicro.com> wrote:
> At this moment, and apparently since ever, we have no way of enabling
> RISCV_FEATURE_MISA. This means that all the code from write_misa(), all
> the nuts and bolts that handles how to write this CSR, has always been a
> no-op as well because write_misa() will always exit earlier.
> This seems to be benign in the majority of cases. Booting an Ubuntu
> 'virt' guest and logging all the calls to 'write_misa' shows that no
> writes to MISA CSR was attempted. Writing MISA, i.e. enabling/disabling
> RISC-V extensions after the machine is powered on, seems to be a niche
> use.
> It is important to mention that the spec says that MISA is a WARL (Write
> Any Read Legal) CSR, and having the write operations as a no-op is a
> valid spec implementation. Allowing the dormant code to write MISA can
> cause tricky bugs to solve later on. Given that we don't have a
> particularly interesting case of writing MISA to support today, the
> risks outweights the benefits.

typo, outweigh?

> Let's make it official and erase all the body of write_misa(), making it
> an official no-op.
> Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> ---
>  target/riscv/csr.c | 55 ----------------------------------------------
>  1 file changed, 55 deletions(-)

Reviewed-by: Bin Meng <bmeng@tinylab.org>

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