Re: [PATCH v2 09/12] x86/altp2m: add remaining support routines.

From: "Lengyel, Tamas" <tlengyel@novetta.com>
To: Ed White <edmund.h.white@intel.com>
Cc: Ravi Sahita <ravi.sahita@intel.com>,
	Wei Liu <wei.liu2@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>,
	Xen-devel <xen-devel@lists.xen.org>,
	Jan Beulich <jbeulich@suse.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Daniel De Graaf <dgdegra@tycho.nsa.gov>
Subject: Re: [PATCH v2 09/12] x86/altp2m: add remaining support routines.
Date: Thu, 25 Jun 2015 17:33:30 -0400	[thread overview]
Message-ID: <CAD33N+57VemCAjFAqJuyTNzQ+0y5yAQhnvbhb7pua07UVeRTyA@mail.gmail.com> (raw)
In-Reply-To: <558C644A.7080300@intel.com>

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On Thu, Jun 25, 2015 at 4:27 PM, Ed White <edmund.h.white@intel.com> wrote:

> On 06/25/2015 10:42 AM, Lengyel, Tamas wrote:
> > On Thu, Jun 25, 2015 at 12:31 PM, Ed White <edmund.h.white@intel.com>
> wrote:
> >
> >> On 06/24/2015 07:44 PM, Lengyel, Tamas wrote:
> >>>> +    if ( altp2m_active )
> >>>> +    {
> >>>> +        if ( altp2mhvm_hap_nested_page_fault(v, gpa, gla, npfec,
> &p2m)
> >> ==
> >>>> 1 )
> >>>> +        {
> >>>> +            /* entry was lazily copied from host -- retry */
> >>>>
> >>>
> >>> So I'm not fully following this logic here. I can see that the altp2m
> >> entry
> >>> got copied from the host. Why is there a need for the retry, why not
> just
> >>> continue?
> >>
> >> At this point the EPT's that the hardware is using have been made valid
> >> by software, but the hardware has already failed the access so you have
> >> to restart the operation. This isn't in any way specific to altp2m,
> >> it's how page fault logic works generally.
> >>
> >> Ed
> >>
> >
> > Oh I see, you are working with the assumption that the fault was
> triggered
> > by the entry not being present in the altp2m EPT, thus it's enough to
> copy
> > it to resolve the fault. However, if the hostp2m permissions are
> > restricted, there will be a follow-up fault again. Would it maybe make
> > sense to check for that condition and save having to hit two faults?
>
> It's not an assumption, it's a fact because the altp2m nested page fault
> handler returns 1 IFF it has copied from the host p2m.
>
> Once again this is standard page fault handling. Preemptively checking
> for a condition that would cause another fault shortens the path for
> cases that would re-fault, but lengthens it for all the cases that would
> not. In a typical scenario (which your current experiments are not) you
> expect most cases not to re-fault. The cases that do re-fault are much
> more expensive anyway.
>
> There are other reasons not to preemptively check, but that's the most
> straightforward one.
>
> Ed
>

OK, thanks, makes sense.

Tamas

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