Re: [PATCH][RFC] pte notifiers -- support for external page tables

[Avi Kivity <avi@qumranet.com>]

Shaohua Li wrote:
> On Wed, 2007-09-05 at 22:32 +0300, Avi Kivity wrote:
>   
> > [resend due to bad alias expansion resulting in some recipients
> >  being bogus]
> >
> > Some hardware and software systems maintain page tables outside the normal
> > Linux page tables, which reference userspace memory.  This includes
> > Infiniband, other RDMA-capable devices, and kvm (with a pending patch).
> >
> > Because these systems maintain external page tables (and external tlbs),
> > Linux cannot demand page this memory and it must be locked.  For kvm at
> > least, this is a significant reduction in functionality.
> >
> > This sample patch adds a new mechanism, pte notifiers, that allows drivers
> > to register an interest in a changes to ptes. Whenever Linux changes a
> > pte, it will call a notifier to allow the driver to adjust the external
> > page table and flush its tlb.
> >
> > Note that only one notifier is implemented, ->clear(), but others should be
> > similar.
> >
> > pte notifiers are different from paravirt_ops: they extend the normal
> > page tables rather than replace them; and they provide high-level
> > information
> > such as the vma and the virtual address for the driver to use.
> >     
> Looks great. So for kvm, all guest pages will be vma mapped?
> There are lock issues in kvm between kvm lock and page lock. 
>   

Yes, locking will be a headache.

> Will shadow page table be still stored in page->private? If yes, the
> page->private must be cleaned before add_to_swap.
>   

page->private can be in use by filesystems, so we will need to move rmap 
somewhere else.

--
Any sufficiently difficult bug is indistinguishable from a feature.

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