Re: [PATCH 0/4] xen: map foreign pages for shared rings by updating the PTEs directly

[David Vrabel <david.vrabel@citrix.com>]

On 29/09/11 17:07, Jan Beulich wrote:
>>>> On 29.09.11 at 17:53, David Vrabel <david.vrabel@citrix.com> wrote:
>> [Resend as requested by Konrad.]
>>
>> This series of patches allows the vmalloc_sync_all() to be removed
>> from alloc_vm_area() by getting the hypervisor to update the PTEs (in
>> init_mm) directly rather than having the hypervisor look in the
>> current page tables to find the PTEs.
>>
>> Once the hypervisor has updated the PTEs, the normal mechanism of
>> syncing the page tables after a fault works as expected.
> 
> Did you actually test that, and namely the case where alloc_vm_area()
> would result in a new top level page directory entry to get populated?
>
> I cannot see how this new entry would propagate into other mm-s, and
> hence I cannot see how you can do away with calling vmalloc_sync_all()
> just by changing how leaf page table entries get populated.

I don't think this new behaviour of alloc_vm_area() is any different
from how a regular vmalloc() works.

vmalloc_fault() copies the page table entries from init_mm into the
current MM (on 32-bit it calls vmalloc_sync_one() which makes it
obviously correct I think).

David

>> This mechanism doesn't currently work on the ia64 port as that does
>> not support the GNTMAP_contains_pte flag.
>>
>> David