lguest: use get_user_pages_fast() instead of get_user_pages()
Using a simple page table thrashing program I measure a slight
improvement. The program creates five processes. Each touches 1000
pages then schedules the next process. We repeat this 1000 times. As
lguest only caches 4 cr3 values, this rebuilds a lot of shadow page
tables requiring virt->phys mappings.
Before: 5.93 seconds
After: 5.40 seconds
(Counts of slow vs fastpath in this usage are 6092 and 2852462 respectively.)
And more importantly for lguest, the code is simpler.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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