On Fri, Sep 4, 2020 at 9:33 PM Christoph Hellwig <h...@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 <h...@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
