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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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
> time
> > 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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