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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.02.2023 13:16, Matias Ezequiel Vara Larsen wrote:
> On Fri, Feb 17, 2023 at 03:10:53PM +0100, Jan Beulich wrote:
>> 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.
> 
> Thanks. If I understand correctly, _get_page_type() needs to avoid to call
> validate_page() when type = PGT_none.

Yes.

> For the writable and shared pages, this
> is avoided by setting nx |= PGT_validated. Am I right?

Well, no, I wouldn't describe it like that. The two (soon three) types not
requiring validation simply set the flag without calling validate_page().

Jan