Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

[H. Peter Anvin]

Andi, do you happen to remember the details on this?

	-hpa


Linus Torvalds wrote:
> On Thu, 29 Nov 2007, H. Peter Anvin wrote:
> > Linus Torvalds wrote:
> > > > It is advantageous for user space to use the register the kernel typically
> > > > won't, in order to speed up system call entry/exit.
> > > but I'm not seeing the reason for that one. Care to comment more? (Yes,
> > > there is often a latency from segment reload to use, but the reload latency
> > > for system call exit *should* be entirely covered by the cost of doing the
> > > system call return itself, no?)
> > I do seem to recall that some processor implementations can load a NULL
> > segment faster than a non-NULL segment.  This was significant enough that we
> > wanted to use %fs in x86-64 userspace, as opposed to the original ABI which
> > used %gs both in userspace and in the kernel.
>
> Ahh, I think you may be right for some CPUs. The zero selector is indeed 
> potentially faster to load, since it doesn't have to even bother looking 
> at the GDT/LDT.
>
> That said, I doubt it's very noticeable. I just ran tests on both an old 
> P4 and on a more modern Core 2 machine, and for both of those the 
> performance was identical between loading a NUL selector and loading it 
> with a non-zero one.
>
> But I could well imagine that it matters a few cycles on other CPU's. But 
> from my testing, it definitely isn't noticeable, and I think the 
> maintenance advantage of using the same segment setup would more than make 
> up for the fact that maybe some odd CPU can see a difference.
>
> 			Linus
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/