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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan