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

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

On Mon, Jun 11, 2012 at 11:23:11AM +0100, David Vrabel wrote:
> 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@xxxxxxxxxx>
> >>>
> >>> 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?

During the bootup. I hadn't really done much investigation - but reverting
your patch (so v3.5-rc2+stable/for-linus-3.5 minus your patch) makes these
errors go away.
> 
> 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.

The boot doesn't finish. It keeps on printing those forever. This is
of course dom0 - I hadn't gotten to trying out a domU guest.

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