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

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Message-ID: <ZPoMoVnUjNzr+OXI@ghost>
Date:   Thu, 7 Sep 2023 10:47:13 -0700
From:   Charlie Jenkins <charlie@...osinc.com>
To:     Conor Dooley <conor@...nel.org>
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 Thu, Sep 07, 2023 at 10:52:35AM +0100, Conor Dooley wrote:
> 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.
I got caught up in the other patch where I did not want to fall back, so
then in this patch I forgot I could use riscv_has_extension_likely. I will
make the change.

- Charlie