Re: [PATCH 07/17] xen/x86: traps: Convert __page_fault_type() to use typesafe MFN

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.04.2020 13:43, Julien Grall wrote:
> On 18/04/2020 12:01, Julien Grall wrote:
>> On 26/03/2020 15:54, Jan Beulich wrote:
>>> On 22.03.2020 17:14, julien@xxxxxxx wrote:
>>>> From: Julien Grall <jgrall@xxxxxxxxxx>
>>>>
>>>> Note that the code is now using cr3_to_mfn() to get the MFN. This is
>>>> slightly different as the top 12-bits will now be masked.
>>>
>>> And here I agree with the change. Hence it is even more so important
>>> that the patch introducing the new helper(s) first gets sorted.
>>> Should there be further patches in this series with this same
>>> interaction issue, I won't point it out again and may not respond at
>>> all if I see no other issues.
>>
>> I will update the commit message explaining the reason of using cr3_to_mfn() 
>> and look at the other user.
> 
> Looking at the code again, there are a few users that don't mask the top 
> 12-bits. I am trying to understand why this has never been an issue so far.
> 
> Wouldn't it break when bit 63 (no flush) is set?

Yes; I guess those uses are case where bit 63 can't / won't be set.
Just like the register, which doesn't store the bit, I think we
avoid storing the bit set as well. But correctness of the non-
masking variants can only be established by looking at every
individual site.

> If so, maybe I should split the work from typesafe.

Maybe better indeed.

Jan