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

[Chengyuan Li <chengyuanli@xxxxxxxxx>]

I misundstand the writable pagetable mechanism before.
According to your and Ian's explication, I think the performance
wouldn't be affected much.

Could you please tell me where can I obtain some papers that describe
the writable pagetable? Thanks in advance!

Chengyuan


On Sun, 10 Oct 2004 13:00:57 +0100, Mark A. Williamson
<mark.williamson@xxxxxxxxxxxx> wrote:
> > As the queue is canceled, it seems that the performance will decline.
> > Right?
> 
> Writeable pagetable performance is (I believe) comparable to the old batching
> interface.
> 
> Basically, when a guest tries to write to a page table entry, Xen removes that
> node from the page tables and makes it writeable.  The guest can then write
> to it as much as it wants but when it tries to actually *use* that part of
> the page table (i.e. access a page corresponsing to one of its entries)
> another fault occurs and Xen validates the node before returning it (assuming
> it's valid) to the page tables.
> 
> Because the page table node is removed from the table and made writeable, only
> the first change requires a fault.  The node is only restored when it is
> needed, at which point you would have needed a flush anyway in the old
> system.  This ensures good batching happens but it is always safe.
> 
> The end result is pretty much the same performance but the batching happens
> implicitly without the guest having to worry about it.  This makes porting
> simpler, requires less source code changes and hopefully less tedious
> debugging of guest memory management ;-)
> 
> HTH,
> Mark
> 
> 
> 
> > 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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