Andi Kleen wrote:
I bet qemu doesn't have a real descriptor cache unlike real CPUs.
So likely it is some disconnect between changing the backing GDT
and referencing the register. Reload %gs more aggressively?
The GDT only gets touched once in cpu_init(), and %gs is reloaded on
every kernel entry, so I don't think that's it. I seems to have
interrupt issues with SMP.
And either way, it still doesn't work on real hardware...
Comparing with SimNow! (which should behave more like a real CPU)
might be also interesting.
Yeah, I'll have to try that out.
- Measure performance impact. The patch adds a segment register
save/restore on entry/exit to the kernel. This expense should be
offset by savings in using the PDA while in the kernel, but I haven't
measured this yet. Space savings are already appealing though.
- Modify more things to use the PDA. The more that uses it, the more
the cost of the %gs save/restore is amortized. smp_processor_id and
current are the obvious first choices, which are implemented in this
series.
per cpu data would be the prime candidate. It is pretty simple.
Well, it has to be arch-specific per-cpu data, since the PDA is arch
specific. But there should be various pieces of interrupt state that
adapt well to it.
- Make it a config option? UP systems don't need to do any of this,
other than having a single pre-allocated PDA. Unfortunately, it gets
a bit messy to do this given the changes needed in handling %gs.
Please don't.
Yeah, that wasn't really a serious thought...
J
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