linux-kernel - Re: [PATCH 13/15] riscv: clear the instruction cache and all registers when booting

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Date:   Wed, 14 Aug 2019 09:00:14 +0800
From:   Alan Kao <alankao@...estech.com>
To:     Christoph Hellwig <hch@....de>
CC:     Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Damien Le Moal <damien.lemoal@....com>,
        <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 13/15] riscv: clear the instruction cache and all
 registers when booting

Hi Christoph,

On Tue, Aug 13, 2019 at 05:47:45PM +0200, Christoph Hellwig wrote:
> When we get booted we want a clear slate without any leaks from previous
> supervisors or the firmware.  Flush the instruction cache and then clear
> all registers to known good values.  This is really important for the
> upcoming nommu support that runs on M-mode, but can't really harm when
> running in S-mode either.  

Sure.

> +#ifdef CONFIG_FPU

But it doesn't really mean that the running system has an FPU given CONFIG_FPU
enabled.  Normally the existence of an FPU is checked in riscv_fill_hwcap by
searching device tree, where the code looks like


bool has_fpu = false; // this one is global
...
#ifdef CONFIG_FPU
        if (elf_hwcap & (COMPAT_HWCAP_ISA_F | COMPAT_HWCAP_ISA_D))
                has_fpu = true;
#endif


Or does CONFIG_FPU have a more intuitive meaning when CONFIG_M_MODE is enabled?

> +	csrr	t0, CSR_MISA
> +	andi	t0, t0, (COMPAT_HWCAP_ISA_F | COMPAT_HWCAP_ISA_D)
> +	bnez	t0, .Lreset_regs_done
> +
> +	li	t1, SR_FS
> +	csrs	CSR_XSTATUS, t1
> +	fmv.s.x	f0, zero
> +	fmv.s.x	f1, zero
> +	fmv.s.x	f2, zero
> +	fmv.s.x	f3, zero
> +	fmv.s.x	f4, zero
> +	fmv.s.x	f5, zero
> +	fmv.s.x	f6, zero
> +	fmv.s.x	f7, zero
> +	fmv.s.x	f8, zero
> +	fmv.s.x	f9, zero
> +	fmv.s.x	f10, zero
> +	fmv.s.x	f11, zero
> +	fmv.s.x	f12, zero
> +	fmv.s.x	f13, zero
> +	fmv.s.x	f14, zero
> +	fmv.s.x	f15, zero
> +	fmv.s.x	f16, zero
> +	fmv.s.x	f17, zero
> +	fmv.s.x	f18, zero
> +	fmv.s.x	f19, zero
> +	fmv.s.x	f20, zero
> +	fmv.s.x	f21, zero
> +	fmv.s.x	f22, zero
> +	fmv.s.x	f23, zero
> +	fmv.s.x	f24, zero
> +	fmv.s.x	f25, zero
> +	fmv.s.x	f26, zero
> +	fmv.s.x	f27, zero
> +	fmv.s.x	f28, zero
> +	fmv.s.x	f29, zero
> +	fmv.s.x	f30, zero
> +	fmv.s.x	f31, zero
> +	csrw	fcsr, 0
> +	/* note that the caller must clear SR_FS */
> +#endif /* CONFIG_FPU */