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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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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