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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.02.2023 10:29, Matias Ezequiel Vara Larsen wrote:
> 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

The caller of the function needs to avoid the call not only for writable
and shared pages, but also for this new case of PGT_none.

Jan