[Xen-devel] Re: Should shadow_fault_fast_fail abort?

[Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>]

On Fri, Dec 15, 2006 at 09:27:45AM +0000, Tim Deegan wrote:
> At 15:42 +1100 on 15 Dec (1166197339), Herbert Xu wrote:
> > I noticed your recent patch turned off PFEC_reserved_bit in case
> > another CPU had already modified a magic PTE.  It seems that it
> > still leaves PFEC_page_present enabled.  This could confuse the
> > guest if the PTE modification is such that the PTE is now present
> > and valid.  In fact the guest may treat it as a protection fault.
> 
> It's just a spurious page fault -- the hardware is happy to throw these
> at operating systems and so am I. :)  

It's only a spurious page fault if PFEC_page_present is clear.
Otherwise the OS (Linux in particular) may treat this as a
protection error which may:

1) Kill a user-space process if in user-mode.
2) Oops the kernel otherwise.

> That does seem cleaner.  Is this a problem you were seeing on a
> particular system, or just a general improvement?

I just happened to be looking at the code :)

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
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