Re: [PATCH V2 01/10] x86/CPU: Expose if cache is inclusive of lower level caches

Reinette Chatre
2019-08-06T20:22:27Z

Hi Borislav,

On 8/6/2019 12:16 PM, Borislav Petkov wrote:
> On Tue, Aug 06, 2019 at 11:53:40AM -0700, Reinette Chatre wrote:
>> In get_prefetch_disable_bits() the platforms that support cache
>> pseudo-locking are hardcoded as part of configuring the hardware
>> prefetch disable bits to use.
>
> Ok, so there is already a way to check pseudo-locking support.

Correct. This check is per platform though.

> Now, why
> do we have to look at cache inclusivity too?

... because some platforms differ in which SKUs support cache
pseudo-locking. On these platforms only the SKUs with inclusive cache
support cache pseudo-locking, thus the additional check.

>
> Your 0/10 mail says:
>
> "Only systems with L3 inclusive cache is supported at this time because
> if the L3 cache is not inclusive then pseudo-locked memory within the L3
> cache would be evicted when migrated to L2."
>
> but then a couple of mails earlier you said:
>
> "... this seems to be different between L2 and L3. On the Atom systems
> where L2 pseudo-locking works well the L2 cache is not inclusive. We are
> also working on supporting cache pseudo-locking on L3 cache that is not
> inclusive."
>
> which leads me to still think that we don't really need L3 cache
> inclusivity and theoretically, you could do without it.
>
> Or are you saying that cache pseudo-locking on non-inclusive L3 is not
> supported yet so no need to enable it yet?

Correct. Hardware and software changes will be needed to support this.

Reinette