linux-kernel - Re: [PATCH v2 2/5] riscv: Add checksum library

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Message-ID: <20230907-9d5d2e7ef2a20edd75514470@fedora>
Date:   Thu, 7 Sep 2023 10:52:35 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Charlie Jenkins <charlie@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Samuel Holland <samuel.holland@...ive.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>
Subject: Re: [PATCH v2 2/5] riscv: Add checksum library

On Tue, Sep 05, 2023 at 09:46:51PM -0700, Charlie Jenkins wrote:

> +#ifdef CONFIG_RISCV_ISA_ZBB
> +	if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
> +		/*
> +		 * Zbb is likely available when the kernel is compiled with Zbb
> +		 * support, so nop when Zbb is available and jump when Zbb is
> +		 * not available.
> +		 */
> +		asm_volatile_goto(ALTERNATIVE("j %l[no_zbb]", "nop",  0,
> +					      RISCV_ISA_EXT_ZBB, 1)
> +		    :
> +		    :
> +		    :
> +		    : no_zbb);
> +	} else {
> +		if (!__riscv_isa_extension_available(NULL, RISCV_ISA_EXT_ZBB))
> +			goto no_zbb;
> +	}

Again, do these constructs have an appreciable benefit over doing
if (!riscv_has_extension_likely(<ZBB>)
	goto no_zbb;

?

That encaspulates the fallback to a non-alternative mechanism for you,
in case you had not noticed.

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