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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.04.2023 21:34, Andrew Cooper wrote:
> On 17/04/2023 3:51 pm, Jan Beulich wrote:
>> 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).
> 
> There's no nice way to check for because a range that starts on a 4k
> non-2M boundary can legitimately end on a 2M boundary at 4k granularity.

How about

        if ( l2e_get_flags(l2e) & _PAGE_PSE )
        {
            ASSERT(l1_table_offset(v) == 0);
            ASSERT(e - v >= (1UL << L2_PAGETABLE_SHIFT));

?

> How about ", i.e. s and e must not be in the middle of a superpage." then?

That sounds good, thanks.

Jan