Re: [PATCH v2 1/3] riscv: Enable cbo.zero only when all harts support Zicboz

[Conor Dooley <conor.dooley@microchip.com>]

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On Thu, Jun 13, 2024 at 10:14:39AM -0700, Samuel Holland wrote:
> Currently, we enable cbo.zero for usermode on each hart that supports
> the Zicboz extension. This means that the [ms]envcfg CSR value may
> differ between harts. Other features, such as pointer masking and CFI,
> require setting [ms]envcfg bits on a per-thread basis. The combination
> of these two adds quite some complexity and overhead to context
> switching, as we would need to maintain two separate masks for the
> per-hart and per-thread bits. Andrew Jones, who originally added Zicboz
> support, writes[1][2]:
> 
>   I've approached Zicboz the same way I would approach all
>   extensions, which is to be per-hart. I'm not currently aware of
>   a platform that is / will be composed of harts where some have
>   Zicboz and others don't, but there's nothing stopping a platform
>   like that from being built.
> 
>   So, how about we add code that confirms Zicboz is on all harts.
>   If any hart does not have it, then we complain loudly and disable
>   it on all the other harts. If it was just a hardware description
>   bug, then it'll get fixed. If there's actually a platform which
>   doesn't have Zicboz on all harts, then, when the issue is reported,
>   we can decide to not support it, support it with defconfig, or
>   support it under a Kconfig guard which must be enabled by the user.
> 
> Let's follow his suggested solution and require the extension to be
> available on all harts, so the envcfg CSR value does not need to change
> when a thread migrates between harts. Since we are doing this for all
> extensions with fields in envcfg, the CSR itself only needs to be saved/
> restored when it is present on all harts.
> 
> This should not be a regression as no known hardware has asymmetric
> Zicboz support, but if anyone reports seeing the warning, we will
> re-evaluate our solution.
> 
> Link: https://lore.kernel.org/linux-riscv/20240322-168f191eeb8479b2ea169a5e@orel/ [1]
> Link: https://lore.kernel.org/linux-riscv/20240323-28943722feb57a41fb0ff488@orel/ [2]
> Reviewed-by: Deepak Gupta <debug@rivosinc.com>
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>

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