Re: [Xen-devel] [PATCH v6 5/9] x86/mem_sharing: use default_access in add_to_physmap

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.01.2020 15:05, Tamas K Lengyel wrote:
> On Wed, Jan 29, 2020 at 6:27 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 28.01.2020 18:02, Tamas K Lengyel wrote:
>>> On Tue, Jan 28, 2020 at 9:56 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>
>>>> On 27.01.2020 19:06, Tamas K Lengyel wrote:
>>>>> When plugging a hole in the target physmap don't use the access permission
>>>>> returned by __get_gfn_type_access as it can be non-sensical, leading to
>>>>> spurious vm_events being sent out for access violations at unexpected
>>>>> locations. Make use of p2m->default_access instead.
>>>>
>>>> As before, to me "can be non-sensical" is insufficient as a reason
>>>> here. If it can also be a "good" value, it still remains unclear
>>>> why in that case p2m->default_access is nevertheless the right
>>>> value to use.
>>>
>>> I have already explained in the previous version of the patch why I
>>> said "can be". Forgot to change the commit message from "can be" to
>>> "is".
>>
>> Changing just the commit message would be easy while committing.
>> But even with the change I would ask why this is. Looking at
>> ept_get_entry() (and assuming p2m_pt_get_entry() will work
>> similarly, minus the p2m_access_t which can't come out of the
>> PTE just yet), I see
>>
>>     if ( is_epte_valid(ept_entry) )
>>     {
>>         *t = p2m_recalc_type(recalc || ept_entry->recalc,
>>                              ept_entry->sa_p2mt, p2m, gfn);
>>         *a = ept_entry->access;
>>
>> near its end. Which means even a hole can have its access field
>> set. So it's still not clear to me from the description why
>> p2m->default_access is uniformly the value to use. Wouldn't you
>> rather want to override the original value only if it's
>> p2m_access_n together with p2m_invalid or p2m_mmio_dm (but not
>> paged-out pages)?
> 
> At this point I would just rather state that add_to_physmap only works
> on actual holes, not with paged-out pages. In fact, I would like to
> see mem_paging being dropped from the codebase entirely since it's
> been abandoned for years and noone expressing any interest in keeping
> it. In the interim I would rather not spend unnecessary cycles on
> speculating about potential corner-cases of mem_paging when noone
> actually uses it.
> 
>> Of course then the question is whether there
>> wouldn't be an ambiguity with p2m_access_n having got set
>> explicitly on the page. But maybe this is impossible for
>> p2m_invalid / p2m_mmio_dm?
> 
> As far as mem_access permissions go, I don't know of any usecase that
> would set mem_access permission on a hole even if by looks of it it is
> technically possible. At this point I would rather just put this
> corner-case's description in a comment.

I think I would ack a revised patch having this properly explained.

Jan

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