Re: [Xen-devel] [PATCH] x86/ept: pass correct level to p2m_entry_modify

[Jan Beulich <JBeulich@xxxxxxxx>]

On 03.07.2019 13:01, Roger Pau Monné  wrote:
> On Wed, Jul 03, 2019 at 10:22:03AM +0000, Jan Beulich wrote:
>> On 03.07.2019 11:43, Roger Pau Monne wrote:
>>> EPT differs from NPT and shadow when translating page orders to levels
>>> in the physmap page tables. EPT page tables level for order 0 pages is
>>> 0, while NPT and shadow instead use 1, ie: EPT page tables levels
>>> starts at 0 while NPT and shadow starts at 1.
>>>
>>> Fix the p2m_entry_modify call in atomic_write_ept_entry to always add
>>> one to the level, in order to match NPT and shadow usage.
>>>
>>> While there also fix p2m_entry_modify BUG condition to trigger when
>>> foreign or ioreq entries with level different than 0 are attempted.
>>> That should allow to catch future errors related to the level
>>> parameter.
>>>
>>> Fixes: c7a4c0 ('x86/mm: split p2m ioreq server pages special handling into 
>>> helper')
>>
>> A 6-digit hash is definitely too short in the long run. I understand
>> that this then wants backporting to the 4.12 tree.
> 
> Yes.
> 
> Is there consensus on how many digits to use 8, 12, 16?

Consensus - no, I don't think so. But anything below 8 seems
open for collisions in the foreseeable future (albeit I don't
know if git has separate hash name spaces for commits and
objects in general). I've been using 10 for the last so many
years ...

Jan
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