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Re: [Xen-devel] [PATCH] turn off writable page tables

On 28 Jul 2006, at 16:51, Ian Pratt wrote:

So, in summary, we know writable page tables are not broken, they just
don't help on typical workloads because the PTEs/page are so low.
However, they do hurt SMP guest performance.  If we are not seeing a
benefit today, should we turn it off?  Should we make it a compile
option, with the default off?

I wouldn't mind seeing wrpt removed altogether, or at least emulation
made the compile time default for the moment. There's bound to be some
workload that bites us in the future which is why batching updates on
the fork path mightn't be a bad thing if it can be done without too much
gratuitous hacking of linux core code.

My only fear is that batched wrpt has some guest-visible effects. For example, the guest has to be able to cope with seeing page directory entries with the present bit cleared. Also, on SMP, it has to be able to cope with spurious page faults anywhere in its address space (e.g., faults on a unhooked page table which some other VCPU has rehooked by the time the Xen pagefault handler runs, hence the fault is bounced back to the guest even though there is no work to be done). If we turn off batched wrpt then guests will not be tested against it and we are likely to hit problems if we ever want to turn it back on again -- we'll find that some guests are not able to correctly handle the weird side effects.

On the other hand, perhaps we can find a neater more explicit alternative to batched wrpt in future.

 -- Keir

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