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Re: [RFC PATCH v2 1/2] xen/memory : Add a stats_table resource type
- To: Matias Ezequiel Vara Larsen <matiasevara@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 14 Dec 2022 08:56:57 +0100
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- Cc: Matias Ezequiel Vara Larsen <matias.vara@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 14 Dec 2022 07:57:10 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
Jan
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