Re: [Xen-devel] [PATCH 3/3] memory: restrict XENMEM_remove_from_physmap to translated guests

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.04.19 at 11:50, <julien.grall@xxxxxxx> wrote:
> On 08/04/2019 15:29, Jan Beulich wrote:
>>>>> On 08.04.19 at 13:47, <julien.grall@xxxxxxx> wrote:
>>> de-allocation step aside, I am not really convinced you can reuse
>>> guest_remove_page() here. On x86, the function will not work on certain
>>> p2m types. Is it what we really want?
>> 
>> Hmm, I'm confused. Afaics the only two types it refuses a request
>> for are p2m_invalid and p2m_mmio_dm. These represent cases
>> where there's no p2m entry anyway, i.e. nothing to remove. Am
>> I perhaps overlooking something you see?
>> 
>> Or are you referring to the mfn_invalid() check (which isn't x86-
>> specific)? This ought to be covered by the p2m_is_paging() and
>> p2m_mmio_direct special cases a few lines up from there. Other
>> cases with invalid MFNs would indeed represent an error condition
>> imo.
> 
> I am referring to get_page(...) which would not work for foreign pages.

Ah, that's part of the de-allocation portion of the code, which I've
previously indicated would need to be skipped in the case here.

>> In the end it's actually quite the opposite that I'm thinking: For
>> the caller it shouldn't, for example, matter whether the
>> requested page was paged out. We wouldn't even call
>> guest_physmap_remove_page() in that case, while
>> guest_remove_page() would take care of it.
> 
> But those pages should never be removed via physmap, I am correct?

The guest is unaware of paging, so as long as it's permitted to
use this hypercall, it should make no difference to it whether a
page is actually in memory at the time it issues this hypercall.

> Anyway, guest_physmap_remove_page() would need some rework to do the correct 
> things for physmap.

Of course.

> I will wait an see your patch to comment.

Well, the reason for the TBD item in the patch here was precisely
to figure out whether creating such a patch would make sense,
or whether the current use of guest_physmap_remove_page() is
correct.

Jan



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel