Re: [Xen-devel] [EXTERNAL][PATCH v3 2/6] x86 / p2m: remove page_list check in p2m_alloc_table

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 06 March 2020 13:07
> To: Durrant, Paul <pdurrant@xxxxxxxxxxxx>
> Cc: pdurrant@xxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper 
> <andrew.cooper3@xxxxxxxxxx>;
> George Dunlap <george.dunlap@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Roger Pau 
> Monné <roger.pau@xxxxxxxxxx>
> Subject: RE: [EXTERNAL][PATCH v3 2/6] x86 / p2m: remove page_list check in 
> p2m_alloc_table
> 
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> 
> On 06.03.2020 13:50, Durrant, Paul wrote:
> >> -----Original Message-----
> >> From: Jan Beulich <jbeulich@xxxxxxxx>
> >> Sent: 06 March 2020 12:47
> >> To: Durrant, Paul <pdurrant@xxxxxxxxxxxx>
> >> Cc: pdurrant@xxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper 
> >> <andrew.cooper3@xxxxxxxxxx>;
> >> George Dunlap <george.dunlap@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Roger Pau 
> >> Monné
> <roger.pau@xxxxxxxxxx>
> >> Subject: Re: [PATCH v3 2/6] x86 / p2m: remove page_list check in 
> >> p2m_alloc_table
> >>
> >> On 06.03.2020 13:07, Durrant, Paul wrote:
> >>>> -----Original Message-----
> >>>> From: Jan Beulich <jbeulich@xxxxxxxx>
> >>>> Sent: 06 March 2020 11:46
> >>>> To: pdurrant@xxxxxxxx
> >>>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Durrant, Paul 
> >>>> <pdurrant@xxxxxxxxxxxx>; Andrew Cooper
> >>>> <andrew.cooper3@xxxxxxxxxx>; George Dunlap <george.dunlap@xxxxxxxxxx>; 
> >>>> Wei Liu <wl@xxxxxxx>;
> Roger
> >> Pau
> >>>> Monné <roger.pau@xxxxxxxxxx>
> >>>> Subject: Re: [PATCH v3 2/6] x86 / p2m: remove page_list check in 
> >>>> p2m_alloc_table
> >>>>
> >>>> On 05.03.2020 13:45, pdurrant@xxxxxxxx wrote:
> >>>>> From: Paul Durrant <pdurrant@xxxxxxxxxx>
> >>>>>
> >>>>> There does not seem to be any justification for refusing to create the
> >>>>> domain's p2m table simply because it may have assigned pages.
> >>>>
> >>>> I think there is: If any such allocation had happened before, how
> >>>> would it be represented in the domain's p2m?
> >>>
> >>> Insertion into the p2m is a separate action from page allocation. Why 
> >>> should they be linked?
> >>
> >> They are, because of how XENMEM_populate_physmap works. Yes,
> >> they _could_ be separate steps, but that's only a theoretical
> >> consideration.
> >
> > Then surely the check should be in the XENMEM_populate_physmap code?
> 
> How that? populate-physmap can be called any number of times. We
> can't refuse a 2nd call there just because a 1st one had happened
> already. Or did you mean the inverse check (i.e. that there
> already is a p2m)?

Yes, I mean check the p2m has been initialized there.

> This surely wouldn't be a bad idea, as
> otherwise both ept_get_entry() and p2m_pt_get_entry() would
> blindly map MFN 0. But adding such a check wouldn't eliminate
> the reason to also have the check that you're proposing to drop.
> 

Why not? Anywhere assuming the existence of a p2m ought to check for it; I 
still can't see why initialising the p2m after having allocated pages 
(PGC_extra or otherwise) is inherently wrong.

  Paul

> Jan


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