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
next prev parent reply other threads:[~2025-01-27 20:55 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-01-22 22:56 [PATCH 0/4] entry: Move ret_from_fork() to C and inline syscall_exit_to_user_mode() Charlie Jenkins
2025-01-22 22:56 ` [PATCH 1/4] riscv: entry: Convert ret_from_fork() to C Charlie Jenkins
2025-01-24 7:09 ` Alexandre Ghiti
2025-01-24 7:56 ` Charlie Jenkins
2025-01-25 23:30 ` Maciej W. Rozycki
2025-01-26 19:51 ` Charlie Jenkins
2025-01-27 20:20 ` Maciej W. Rozycki
2025-01-27 20:55 ` Charlie Jenkins [this message]
2025-01-22 22:56 ` [PATCH 2/4] riscv: entry: Split ret_from_fork() into user and kernel Charlie Jenkins
2025-01-22 22:56 ` [PATCH 3/4] loongarch: entry: Migrate ret_from_fork() to C Charlie Jenkins
2025-01-23 17:12 ` kernel test robot
2025-01-22 22:56 ` [PATCH 4/4] entry: Inline syscall_exit_to_user_mode() Charlie Jenkins
2025-01-23 17:12 ` kernel test robot
2025-01-23 17:34 ` kernel test robot
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