Re: [RFC PATCH v2 1/2] xen/memory : Add a stats_table resource type

[Matias Ezequiel Vara Larsen <matiasevara@xxxxxxxxx>]

On Fri, Feb 17, 2023 at 09:57:43AM +0100, Jan Beulich wrote:
> On 17.02.2023 09:50, Matias Ezequiel Vara Larsen wrote:
> > On Wed, Dec 14, 2022 at 08:56:57AM +0100, Jan Beulich wrote:
> >> On 14.12.2022 08:29, Jan Beulich wrote:
> >>> On 07.10.2022 14:39, Matias Ezequiel Vara Larsen wrote:
> >>>> +static int stats_vcpu_alloc_mfn(struct domain *d)
> >>>> +{
> >>>> +    struct page_info *pg;
> >>>> +
> >>>> +    pg = alloc_domheap_page(d, MEMF_no_refcount);
> >>>
> >>> The ioreq and vmtrace resources are also allocated this way, but they're
> >>> HVM-specific. The one here being supposed to be VM-type independent, I'm
> >>> afraid such pages will be accessible by an "owning" PV domain (it'll
> >>> need to guess the MFN, but that's no excuse).
> >>
> >> Which might be tolerable if it then can't write to the page. That would
> >> require "locking" the page r/o (from guest pov), which ought to be
> >> possible by leveraging a variant of what share_xen_page_with_guest()
> >> does: It marks pages PGT_none with a single type ref. This would mean
> >> ...
> >>
> >>>> +    if ( !pg )
> >>>> +        return -ENOMEM;
> >>>> +
> >>>> +    if ( !get_page_and_type(pg, d, PGT_writable_page) ) {
> >>
> >> ... using PGT_none here. Afaict this _should_ work, but we have no
> >> precedent of doing so in the tree, and I may be overlooking something
> >> which prevents that from working.
> >>
> > 
> > I do not fully understand this. I checked share_xen_page_with_guest() and I
> > think you're talking about doing something like this for each allocated 
> > page to
> > set them ro from a pv guest pov:
> > 
> > pg->u.inuse.type_info = PGT_none;
> > pg->u.inuse.type_info |= PGT_validated | 1;
> > page_set_owner(page, d); // not sure if this is needed
> > 
> > Then, I should use PGT_none instead of PGT_writable_page in
> > get_page_and_type(). Am I right?
> 
> No, if at all possible you should avoid open-coding anything. As said,
> simply passing PGT_none to get_page_and_type() ought to work (again, as
> said, unless I'm overlooking something). share_xen_page_with_guest()
> can do what it does because the page isn't owned yet. For a page with
> owner you may not fiddle with type_info in such an open-coded manner.
> 

Thanks. I got the following bug when passing PGT_none:

(XEN) Bad type in validate_page 0 t=0000000000000001 c=8040000000000002
(XEN) Xen BUG at mm.c:2643

I did not investigate yet why this has happened.

Matias