[PATCH 7/7] arm64: hugetlb: Add break-before-make logic for contiguous entries

[Punit Agrawal]

On 22/03/17 14:38, Mark Rutland wrote:
> On Tue, Mar 21, 2017 at 06:04:21PM +0000, Punit Agrawal wrote:
>> From: Steve Capper <steve.capper at arm.com>
>>
>> It has become apparent that one has to take special care when modifying
>> attributes of memory mappings that employ the contiguous bit.
>>
>> Both the requirement and the architecturally correct "Break-Before-Make"
>> technique of updating contiguous entries can be found described in:
>> ARM DDI 0487A.k_iss10775, "Misprogramming of the Contiguous bit",
>> page D4-1762.
>>
>> The huge pte accessors currently replace the attributes of contiguous
>> pte entries in place thus can, on certain platforms, lead to TLB
>> conflict aborts or even erroneous results returned from TLB lookups.
>>
>> This patch adds a helper function get_clear_flush(.) that clears a
>> contiguous entry and returns the head pte (whilst taking care to
>> retain dirty bit information that could have been modified by DBM).
>> A tlb invalidate is performed to then ensure that there is no
>> possibility of multiple tlb entries being present for the same
>> region.
>
> Since its evidently easy to miss, can we please add a comment above
> get_clear_flush() regarding the BBM requirement, e.g.
>
> /*
>  * Changing some bits of contiguous entries requires us to follow a
>  * Break-Before-Make approach, breaking the whole contiguous set before
>  * we can change any entries. See ARM DDI 0487A.k_iss10775,
>  * "Misprogramming of the Contiguous bit", page D4-1762.
>  *
>  * This helper performs the break step.
>  */
>
> Otherwise, this looks good to me, and to the best of my knowledge avoids
> the issue described above.
>
> FWIW:
>
> Reviewed-by: Mark Rutland <mark.rutland at arm.com>

Hi Mark,

Thanks for reviewing the patches. I'll address your comments and post a
new version in a day or two.

Thanks,
Punit

>
> Mark.
>
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