Re: [PATCH v2] x86/livepatch: Fix livepatch application when CET is active

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.04.2023 16:41, Andrew Cooper wrote:
> On 17/04/2023 2:59 pm, Jan Beulich wrote:
>> On 17.04.2023 15:52, Andrew Cooper wrote:
>>> @@ -5879,6 +5880,73 @@ int destroy_xen_mappings(unsigned long s, unsigned 
>>> long e)
>>>      return modify_xen_mappings(s, e, _PAGE_NONE);
>>>  }
>>>  
>>> +/*
>>> + * Similar to modify_xen_mappings(), but used by the alternatives and
>>> + * livepatch in weird contexts.  All synchronization, TLB flushing, etc is 
>>> the
>>> + * responsibility of the caller, and *MUST* not be introduced here.
>>> + *
>>> + * Must be limited to XEN_VIRT_{START,END}, i.e. over l2_xenmap[].
>>> + * Must be called with present flags, and over present mappings.
>>> + * Must be called on leaf page boundaries.
>> This last sentence, while wording-wise correct, could do with making more
>> explicit that it is the caller's responsibility to know whether large page
>> mappings are in use, due to ...
> 
> The meaning here is really "this doesn't shatter superpages", and this
> was the most concise I could come up with.
> 
> Would ", i.e. won't shatter 2M pages." as a clarification work?

Yes, that would definitely help. Nevertheless I was more after something
like "..., i.e. for 2M mappings on 2M boundaries." Which, thinking about
it, points out that while you have a respective check for the start
address, the full 2M page would be changed even if the end address wasn't
2M aligned (but fell in the middle of a 2M page).

Jan