linux-kernel - Re: [PATCH RFC 2/2] RISC-V: add T-Head vector errata handling

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Message-Id: <23381fc8-c297-46ed-9e11-a2643e18e4c9@app.fastmail.com>
Date:   Thu, 22 Jun 2023 14:58:37 -0400
From:   "Stefan O'Rear" <sorear@...tmail.com>
To:     "Heiko Stuebner" <heiko@...ech.de>,
        "Palmer Dabbelt" <palmer@...belt.com>
Cc:     linux-riscv@...ts.infradead.org, samuel@...lland.org,
        guoren@...nel.org, christoph.muellner@...ll.eu,
        conor.dooley@...rochip.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 2/2] RISC-V: add T-Head vector errata handling

On Thu, Jun 22, 2023, at 1:39 PM, Heiko Stübner wrote:
> Am Dienstag, 13. Juni 2023, 08:35:53 CEST schrieb Stefan O'Rear:
>> vxrm and vxsat are part of fcsr in 0.7.1, so they should already have been
>> handled by __fstate_save and __fstate_restore, and this code is likely to
>> misbehave (saving the new process's vxrm/vxsat in the old process's save area
>> because float state is swapped before vector state in __switch_to).
>
> I'm not sure I follow your description but may be overlooking or have
> misunderstood something.
>
> Somehow I way to often have trouble resolving CSR addresses, but according
> to openSBI, FCSR has the location of 0x3
> (#define CSR_FCSR 0x003 in include/sbi/riscv_encoding.h)
>
> where CSR_VXSAT and CSR_VXRM are at 0x9 and 0xa respectively.
> (#define CSR_VXSAT 0x9 and  #define CSR_VXRM 0xa)
>
>
> And looking at __fstate_save + __fstate_restore the only CSRs accessed seem
> to be CSR_STATUS and FCSR itself.
>
> I definitly won't claim to be right, but don't see the issue yet.
>
>
> Thanks for a hint
> Heiko

Are you aware of "3.7. Vector Fixed-Point Fields in fcsr" in
riscv-v-spec-0.7.1.pdf?

-s