Re: scalable kmap (was Re: vm lock contention reduction)

[ebiederm@xmission.com (Eric W. Biederman)]

Linus Torvalds <torvalds@transmeta.com> writes:

> Hmm.. Right now we have the same IDT and GDT on all CPU's, so _if_ the CPU
> is stupid enough to do a locked cycle to update the "A" bit on the
> segments (even if it is already set), you would see horrible cacheline
> bouncing for any interrupt.
> 
> I don't know if that is the case. I'd _assume_ that the microcode was
> clever enough to not do this, but who knows. It should be fairly easily
> testable (just "SMOP") by duplicating the IDT/GDT across CPU's.

If you don't carry about the "A" bit and I don't think we do this is
trivial preventable.  You can set when you initialize the GDT/IDT and
it will never be updated.

I had to make this change a while ago in LinuxBIOS because P4's lock up
when you load a GDT from a ROM that doesn't have the accessed bit set.

The fact it doesn't lock up is a fairly good proof that no writes happen
when the accessed bit is already set.

Eric
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