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From:   Anup Patel <anup@brainfault.org>
Date:   Fri, 4 Sep 2020 21:44:55 +0530
Message-ID: <CAAhSdy2RqZg82Q0+ChRKV6h+Ot90q2Qea9h5s=42SEa3Px4gLg@mail.gmail.com>
Subject: Re: [GIT PULL] RISC-V Fixes for 5.9-rc2
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Qiu Wenbo <qiuwenbo@kylinos.com.cn>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Fri, Sep 4, 2020 at 9:33 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Fri, Sep 04, 2020 at 07:26:41PM +0530, Anup Patel wrote:
> > On Fri, Sep 4, 2020 at 6:30 PM Christoph Hellwig <hch@infradead.org> wrote:
> > >
> > > On Fri, Sep 04, 2020 at 08:58:25PM +0800, Qiu Wenbo wrote:
> > > > I can confirm this patch also breaks K210 support. It seems that
> > > > csr_read(CSR_TIME) will trigger an illegal instruction exception on K210.
> > >
> > > CSR_TIME is trapped by just about every implementation I know (which is
> > > explicitly allowed by the spec).  That is why we should never use it
> > > from common M-mode code.
> >
> > Finally, I was able to replicate this issue by manually hacking QEMU to
> > not emulatie TIME CSR for virt machine.
> >
> > It seems this issue is only seen on older QEMU and Kendrtye K210.
>
> You'd also see it when running nommu on Sifivie or just about any
> hardware.  Whoever implement the TIME CSR for qemu made a mistake IMHO
> as it doesn't match how most real hardware behaves.

There are quite a few RISCV systems who implement TIME CSR in
hardware due to performance gains (10+ %).

The QEMU virt machine does not represent real-world HW so we
should emulate all possible HW optimizations in QEMU virt machine.

On other hand, the QEMU sifive_u machine correctly matches the
real-world SiFive Unleashed in-context of TIME CSR and other HW
features.

IMHO, we need nommu defconfig for SiFive Unleashed so that we
can try NoMMU kernel on both QEMU virt and QEMU sifive_u machine.

Regards,
Anup