linux-kernel - Re: [PATCH v3] RISC-V: Fix Maximum Physical Memory 2GiB option for 64bit systems

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Message-ID: <CAAhSdy0k0gSipzne1=agZzTWa0URoq3OH7vefJsr2=SEMyJ1tg@mail.gmail.com>
Date:   Fri, 5 Apr 2019 11:31:11 +0530
From:   Anup Patel <anup@...infault.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <Atish.Patra@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] RISC-V: Fix Maximum Physical Memory 2GiB option for
 64bit systems

On Fri, Apr 5, 2019 at 11:24 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Fri, Apr 05, 2019 at 05:49:34AM +0000, Anup Patel wrote:
> > The Maximum Physical Memory 2GiB option for 64bit systems is currently
> > broken because kernel hangs at boot-time when this option is enabled
> > and the underlying system has more than 2GiB memory.
> >
> > This issue can be easily reproduced on SiFive Unleashed board where
> > we have 8GiB of memory.
> >
> > This patch fixes above issue by removing unusable memory region in
> > setup_bootmem().
> >
> > Signed-off-by: Anup Patel <anup.patel@....com>
> > Reviewed-by: Christoph Hellwig <hch@....de>
>
> Btw, what is the rationale behind even offering the 2GiB option and
> the medlow model on 64-bit?  Do we reall have use cases where the
> slightly more effient generated code matters so much to keep up
> the support burden of this mostly unused and unusual configuration?

Yes, if we want to use medlow with 64bit then max physical memory
can be 2GiB. Otherwise, I don't any practical use of restricting max
physical memory to 2GiB on 64bit systems.

BTW, as-per latest revision of RISC-V priviledge spece the MMU SV32
mode will be able to access 34bit physical memory (i.e. 16GiB) so
max physical memory 2GiB will also be required for 32bit system for
medlow.

Regards,
Anup