On 06/08/2010 05:35 AM, Xiao Guangrong wrote:
We can avoid the exchange in most cases, for example if the new spte has
the accessed bit set (already in the patch set) or if the page is
already marked as accessed, or if we see the old spte has the accessed
bit set (so no race can occur). I'll update the patches to avoid
atomics when possible.
Umm, the reason that we need atomics here is to avoid vcpu to update spte when we read A bit
form it, so, perhaps we can use below way to avoid atomics completely:
- set reserved bit in spte
- get A bit form spte
- set new spte
the worst case is cause vcpu #PF here, but it doesn't matter since the old mapping is already invalid,
also need a remote tlb flush later.
To set the reserved bit in the spte, you need an atomic operation (well,
unless you use a sub-word-acccess to set a reserved bit in the high 32
bits).
I don't think atomics are that expensive, though, ~20 cycles on modern
processors?
Yes, but atomics are "LOCK" instructions, it can stop multiple cpus runing in parallel.
Only if those cpus are accessing the same word you're accessing.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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