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Date:   Fri, 4 Sep 2020 17:03:35 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Anup Patel <anup@brainfault.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        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>
Subject: Re: [GIT PULL] RISC-V Fixes for 5.9-rc2
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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.