Re: [Xen-devel] What's the advantage of Writable pagetable?

[Chengyuan Li <chengyuanli@xxxxxxxxx>]

As the queue is canceled, it seems that the performance will decline. Right?


On Sat, 09 Oct 2004 10:11:26 +0100, Keir Fraser
<keir.fraser@xxxxxxxxxxxx> wrote:
> 
> Advantages are that update snaturally get batched, rather than needong
> to be explicitly maintainbed in a queue and then explicitly
> flushed. The flushes are a problem in Linux 2.4 because they are
> needed in various places in the generic VM code, to avoid races where
> the kernel reads a PTE that it has just written, but where the update
> is still buffered in in the update queue.
> 
> A further advantage is that atomic read-modify-write instructions will
> trivially work correctly with writable page tables. e.g., when we
> support SMP guests they will wish to 'CMPXCHG' the contents of a PTE
> to zero them while notg racing against updates of accessed/dirty bits
> by another CPU. Using the old interface with explicit batching. we
> would have to add a new kind of update command to support each kind of
> atomic instruction!
> 
> -- Keir
> 
> 
> 
> 
> > Hi,
> >
> > Before writable pagetable is enabled, XenLinux use Hypercall to update
> > PTE. After the writable pagetable is enabled, it depends on page fault
> > to update PTE. What's the advantage then?
> >
> >
> > Thanks,
> > Chengyuan
> >
> > 
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