Re: [PATCH] xen/mm: do direct hypercall in xen_set_pte() if batching is unavailable

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

On 10/06/12 11:23, Konrad Rzeszutek Wilk wrote:
> On Tue, Jun 05, 2012 at 12:07:46PM -0400, Konrad Rzeszutek Wilk wrote:
>> On Fri, Jun 01, 2012 at 04:14:54PM +0100, David Vrabel wrote:
>>> From: David Vrabel <david.vrabel@citrix.com>
>>>
>>> In xen_set_pte() if batching is unavailable (because the caller is in
>>> an interrupt context such as handling a page fault) it would fall back
>>> to using native_set_pte() and trapping and emulating the PTE write.
>>>
>>> On 32-bit guests this requires two traps for each PTE write (one for
>>> each dword of the PTE).  Instead, do one mmu_update hypercall
>>> directly.
>>
>> OK.
>>>
>>> This significantly improves page fault performance in 32-bit PV
>>> guests.
>>
>> Nice!
> 
> With this patch I keep on getting this (which is v3.5-rc2 plus my
> patches in stable/for-linus-3.5 and yours):
[...]
> (XEN) mm.c:659:d0 Could not get page ref for pfn fffffffffffff
> (XEN) mm.c:3460:d0 Could not get page for normal update

Are you talking about these?  I've not seen them.  Do you know when they
happen?

The patch doesn't change what PTEs are written or their value so I don't
think I've introduced a regression -- only it now prints a new
warning/error.

David