Linux-kernel mailing list archive 2000-40,: Re: wasting time on page fault

# Re: wasting time on page fault

Andi Kleen (ak@suse.de)
Wed, 4 Oct 2000 17:11:46 +0200

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On Wed, Oct 04, 2000 at 11:00:41AM -0400, Brian Gerst wrote:
> Mikulas Patocka wrote:
> >
> > Hi.
> >
> > arch/i386/kernel/entry.S
> > xchgl %eax, ORIG_EAX(%esp) # orig_eax (get the error code. )
> > movl %esp,%edx
> > xchgl %ecx, ES(%esp) # get the address and save es.
> > pushl %eax # push the error code
> >
> > xchg with memory operand has implicit lock prefix and is slooooooow. It is
> > really bad idea to have it in page fault handler.
>
> Thank the small register set of the x86 for this. If you can come up
> with a way to do it in fewer cycles and preserve all the registers and
> stack layout, your input is welcome.

You don't need to preserve any registers which are not %eax, they are already
saved. There are tons of ways to do it via the stack or with a temporary
register.

-Andi

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