Re: [PATCH 1/2] mips: convert syscall to generic entry

From: teng sterling <sterlingteng@gmail.com>
To: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: 陈飞扬 <chris.chenfeiyang@gmail.com>,
	"Jiaxun Yang" <jiaxun.yang@flygoat.com>,
	tglx@linutronix.de, peterz@infradead.org, luto@kernel.org,
	arnd@arndb.de, "Feiyang Chen" <chenfeiyang@loongson.cn>,
	linux-mips@vger.kernel.org, linux-arch@vger.kernel.org,
	"Huacai Chen" <chenhuacai@kernel.org>,
	"Yanteng Si" <siyanteng@loongson.cn>
Subject: Re: [PATCH 1/2] mips: convert syscall to generic entry
Date: Mon, 13 Sep 2021 09:35:19 +0800	[thread overview]
Message-ID: <CAMU9jJo66J_53pjdvH-a4KD_ar6L_MPYnPpT8cp0UhOXp-Gpyg@mail.gmail.com> (raw)
In-Reply-To: <20210908085150.GA5622@alpha.franken.de>

Thomas Bogendoerfer <tsbogend@alpha.franken.de> 于2021年9月8日周三 下午5:36写道：
>
> On Wed, Sep 08, 2021 at 10:08:47AM +0800, 陈飞扬 wrote:
> > On Tue, 7 Sept 2021 at 21:49, Jiaxun Yang <jiaxun.yang@flygoat.com> wrote:
> > >
> > >
> > > 在 2021/9/7 14:16, FreeFlyingSheep 写道:
> > > > From: Feiyang Chen <chenfeiyang@loongson.cn>
> > > >
> > > > Convert mips syscall to use the generic entry infrastructure from
> > > > kernel/entry/*.
> > > >
> > > > There are a few special things on mips:
> > > >
> > > > - There is one type of syscall on mips32 (scall32-o32) and three types
> > > > of syscalls on mips64 (scall64-o32, scall64-n32 and scall64-n64). Now
> > > > convert to C code to handle different types of syscalls.
> > > >
> > > > - For some special syscalls (e.g. fork, clone, clone3 and sysmips),
> > > > save_static_function() wrapper is used to save static registers. Now
> > > > SAVE_STATIC is used in handle_sys before calling do_syscall(), so the
> > > > save_static_function() wrapper can be removed.
> > > >
> > > > - For sigreturn/rt_sigreturn and sysmips, inline assembly is used to
> > > > jump to syscall_exit directly for skipping setting the error flag and
> > > > restoring all registers. Now use regs->regs[27] to mark whether to
> > > > handle the error flag and always restore all registers in handle_sys,
> > > > so these functions can return normally as other architecture.
> > >
> > > Hmm, that would give us overhead of register context on these syscalls.
> > >
> > > I guess it's worthy?
> > >
> >
> > Hi, Jiaxun,
> >
> > Saving and restoring registers against different system calls can be
> > difficult due to the use of generic entry.
> > To avoid a lot of duplicate code, I think the overhead is worth it.
>
> could you please provide numbers for that ? This code still runs
> on low end MIPS CPUs for which overhead might mean a different
> ballpark than some highend Loongson CPUs.
It shows ~2% regression for UnixBench on LOONGSON 3a4000 (a1901).

Thanks,

Yanteng