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

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

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

No -- when a pagetable page is 'unhooked' the guest can write to it
freely without incurring further faults. We validate all the entries and
re-hook the page either when a TLB flush occurs, or when we detect a
page fault on the area of VA space the page maps.

The performance is very similar to the batched interface used in 2.4,
but its less invasive to the guest OS an will make SMP guests easier.

Ian

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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