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

Avi Kivity
2007-09-06T08:38:20Z

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.