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xen-devel
[Xen-devel] Re: Question about x86/mm/gup.c's use of disabled interrupts
Jeremy Fitzhardinge wrote:
Avi Kivity wrote:
Hm, awkward if flush_tlb_others doesn't IPI...
How can it avoid flushing the tlb on cpu [01]? It's it's
gup_fast()ing a pte, it may as well load it into the tlb.
xen_flush_tlb_others uses a hypercall rather than an IPI, so none of
the logic which depends on there being an IPI will work.
Right, of course, that's what we were talking about. I thought
optimizations to avoid IPIs if an mm never visited a cpu.
Simplest fix is to make gup_get_pte() a pvop, but that does seem
like putting a red flag in front of an inner-loop hotspot, or
something...
The per-cpu tlb-flush exclusion flag might really be the way to go.
I don't see how it will work, without changing Xen to look at the flag?
local_irq_disable() is used here to lock out a remote cpu, I don't
see why deferring the flush helps.
Well, no, not deferring. Making xen_flush_tlb_others() spin waiting
for "doing_gup" to clear on the target cpu. Or add an explicit notion
of a "pte update barrier" rather than implicitly relying on the tlb
IPI (which is extremely convenient when available...).
Pick up a percpu flag from all cpus and spin on each? Nasty.
You could use the irq enabled flag; it's available and what native spins
on (but also means I'll need to add one if I implement this).
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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