Re: [patch 3/9] Guest page hinting: volatile page cache.

Andy Whitcroft
2006-09-01T15:48:42Z

Martin Schwidefsky wrote:
> On Fri, 2006-09-01 at 07:57 -0700, Dave Hansen wrote:
>>> +#define PG_state_change 21 /* HV page state is changing. */
>>> +#define PG_discarded 22 /* HV page has been discarded. */
>> We're already desperately short on page flags on 32-bit architectures.
>> It seems a wee bit silly to add two arch-generic flags for what is a
>> very specialized arch-specific feature at this point.
>
> There are even three additional page flags if you apply the full set of
> patches.
>
>> I know that there are 32-bit s390 kernels, but would this be a
>> reasonable feature to restrict to only 64-bit kernels? That might be a
>> decent compromise.
>
> Yes, it is definitly an option to make this a 64-bit only features. In
> particular because the ESSA instruction that is used on s390 is only
> available in zarch mode (=64 bit).

Wow. Well there are only 7 extra bits available in 64 bit mode (the
FIELDS area is larger on 64bit machines). Do we really, really need
three new bits. What are they being used for here.

-apw