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Date: Wed, 4 Dec 2002 14:42:36 -0800 (PST)
From: Linus Torvalds <torvalds@transmeta.com>
To: george anzinger <george@mvista.com>
cc: "David S. Miller" <davem@redhat.com>, <dan@debian.org>,
       <sfr@canb.auug.org.au>, <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH] compatibility syscall layer (lets try again)
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On Wed, 4 Dec 2002, george anzinger wrote:
> 
> On the PARISC I did this (a long time ago in a far away
> place) by unwinding the stack to pick up the registers that
> were saved along the way.  Is this at all feasible?

No. Alpha (and apparently sparc) simply do not save the registers that the
signal handling wants on the stack _at_all_. There are too many registers 
to save at each system call entry point, and 99% of all system calls never 
need it.

The system call return that checks for signals anyway will end up saving a
special stack frame when needed. As will the special signal-related system
calls (sigsuspend() and friends). All of this is not only architecture-
dependent, it is literally coded in assembly language for the
architectures.  See "do_switch_stack()" for alpha.

Anyway, if you wondered why Linux beats every other Unix out there on 
system call overhead, now you know.  And yes, this is important.

		Linus

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