Re: [PATCH 1/4] riscv: entry: Convert ret_from_fork() to C

From: Charlie Jenkins <charlie@rivosinc.com>
To: "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	Alexandre Ghiti <alexghiti@rivosinc.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	loongarch@lists.linux.dev
Subject: Re: [PATCH 1/4] riscv: entry: Convert ret_from_fork() to C
Date: Mon, 27 Jan 2025 12:55:40 -0800	[thread overview]
Message-ID: <Z5fyzMjj_5gqOupB@ghost> (raw)
In-Reply-To: <alpine.DEB.2.21.2501272017140.56843@angie.orcam.me.uk>

On Mon, Jan 27, 2025 at 08:20:56PM +0000, Maciej W. Rozycki wrote:
> On Sun, 26 Jan 2025, Charlie Jenkins wrote:
> 
> > >  Wouldn't it make sense to save a jump here and make it a tail call, i.e.:
> > > 
> > > 	la ra, ret_from_exception
> > > 	tail ret_from_fork
> > > 
> > 
> > I don't believe so due to the return address stack. It was shown in this
> > patch [1] that a 7% performance improvement can be seen on existing
> > riscv hardware by performing the extra jump. Doing tail calls should be
> > avoided on riscv since the hardware can be expected to predict the
> > return address incorrectly every time if the return address is manually
> > changed.
> 
>  Fair enough, thanks.  Though no link to said patch given.

Oops! Here it is:
https://lore.kernel.org/linux-riscv/20240607061335.2197383-1-cyrilbur@tenstorrent.com/.

> 
>   Maciej

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