Re: [Xen-devel] [PATCH for-4.12 2/8] amd/ntp: remove assert that prevents creating 2M MMIO entries

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 2/5/19 1:38 PM, Roger Pau Monné wrote:
> On Tue, Feb 05, 2019 at 05:44:14AM -0700, Jan Beulich wrote:
>>>>> On 05.02.19 at 11:40, <roger.pau@xxxxxxxxxx> wrote:
>>> On Tue, Feb 05, 2019 at 12:45:56AM -0700, Jan Beulich wrote:
>>>>>>> On 04.02.19 at 18:18, <roger.pau@xxxxxxxxxx> wrote:
>>>>> On Mon, Feb 04, 2019 at 09:56:22AM -0700, Jan Beulich wrote:
>>>>>>>>> On 30.01.19 at 11:36, <roger.pau@xxxxxxxxxx> wrote:
>>>>>>> The assert was originally added to make sure that higher order
>>>>>>> regions (> PAGE_ORDER_4K) could not be used to bypass the
>>>>>>> mmio_ro_ranges check performed by p2m_type_to_flags.
>>>>>>>
>>>>>>> This however is already checked in set_mmio_p2m_entry, which makes
>>>>>>> sure that higher order mappings don't overlap with mmio_ro_ranges,
>>>>>>> thus allowing the creation of high order MMIO mappings safely.
>>>>>>
>>>>>> Well, the assertions were added to make sure no other code
>>>>>> path appears that violates this requirement. Arguably e.g.
>>>>>> set_identity_p2m_entry() could gain an order parameter and
>>>>>> then try to establish larger p2m_mmio_direct entries.
>>>>>>
>>>>>> Don't get me wrong, I don't object to the removal of the
>>>>>> assertions, but the description makes it sound as if they were
>>>>>> entirely redundant. Even better would be though if they
>>>>>> could be extended to keep triggering in "bad" cases.
>>>>>
>>>>> I could add something like:
>>>>>
>>>>> ASSERT(!rangeset_overlaps_range(mmio_ro_ranges, mfn_x(mfn),
>>>>>                                 mfn_x(mfn) + PFN_DOWN(MB(2))));
>>>>>
>>>>> I think this should be safe and would trigger in case of misuse.
>>>>
>>>> Looks okay, if slightly extended (or made conditional) to exclude
>>>> the addition of MB(2) to MFN_INVALID to wrap and potentially
>>>> hit a r/o range in the low 1Mb.
>>>
>>> Ack, so it would be:
>>>
>>> ASSERT(mfn_eq(mfn, INVALID_MFN) ||
>>>        !rangeset_overlaps_range(mmio_ro_ranges, mfn_x(mfn),
>>>                                 mfn_x(mfn) + PFN_DOWN(MB(2))));
>>
>> But that's still dropping the other aspect of the original ASSERT():
>>
>>>>>>> --- a/xen/arch/x86/mm/p2m-pt.c
>>>>>>> +++ b/xen/arch/x86/mm/p2m-pt.c
>>>>>>> @@ -668,7 +668,6 @@ p2m_pt_set_entry(struct p2m_domain *p2m, gfn_t 
>>>>>>> gfn_, mfn_t mfn,
>>>>>>>          }
>>>>>>>  
>>>>>>>          ASSERT(p2m_flags_to_type(flags) != p2m_ioreq_server);
>>>>>>> -        ASSERT(!mfn_valid(mfn) || p2mt != p2m_mmio_direct);
>>
>> It also made sure that "valid" MFNs can't be used for mappings with
>> p2m_mmio_direct type. Except that I realize now that this is wrong in
>> certain cases, because MMIO pages may actually have "valid" MFNs.
>> mfn_valid(), after all, only tells us whether there's a struct page_info
>> for the MFN. I wonder if it's really this brokenness that you hit,
>> rather than what is explained in the description.
>>
>> When the assertion was introduced, MMIO wasn't handled by the
>> code correctly anyway (!mfn_valid() MFNs would not have got any
>> mappings at all in the 2M and 1G paths), whereas now we have
>> p2m_allows_invalid_mfn() there. So the situation has become worse
>> with other nearby changes. As a result I think we want to correct
>> the assertion here alongside the addition of what you suggest
>> above. What about
>>
>>     if ( p2mt != p2m_mmio_direct )
>>         ASSERT(mfn_valid(mfn) || (mfn_eq(mfn, INVALID_MFN) &&
>>                p2m_allows_invalid_mfn(p2mt)));
>>     else
>>         ASSERT(!mfn_eq(mfn, INVALID_MFN) &&
>>                !rangeset_overlaps_range(mmio_ro_ranges, mfn_x(mfn),
>>                                         mfn_x(mfn) + PFN_DOWN(MB(2))));

FWIW I agree with this approach (asserting !overlaps for p2m_mmio_direct
types).

 -George

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