Hi,
What is the intention behind icache_is_aliasing on big.LITTLE systems
where some icaches are VIPT and others are PIPT? Is it meant to be
conservative in some sense or should it be made per-CPU?
Thanks for your help,
Salman
Hi,
On Wed, May 08, 2019 at 11:45:03AM -0700, Salman Qazi wrote:
> What is the intention behind icache_is_aliasing on big.LITTLE systems
> where some icaches are VIPT and others are PIPT? Is it meant to be
> conservative in some sense or should it be made per-CPU?
It needs to cover the worst case scenario across all CPUs, i.e. aliasing
VIPT if one of the CPUs has this. We can't make it per-CPU because a
thread performing cache maintenance might be migrated to another CPU
with different cache policy (e.g. sync_icache_aliases()).
--
Catalin
Thank you.
On Thu, May 9, 2019 at 1:50 AM Catalin Marinas <[email protected]> wrote:
>
> Hi,
>
> On Wed, May 08, 2019 at 11:45:03AM -0700, Salman Qazi wrote:
> > What is the intention behind icache_is_aliasing on big.LITTLE systems
> > where some icaches are VIPT and others are PIPT? Is it meant to be
> > conservative in some sense or should it be made per-CPU?
>
> It needs to cover the worst case scenario across all CPUs, i.e. aliasing
> VIPT if one of the CPUs has this. We can't make it per-CPU because a
> thread performing cache maintenance might be migrated to another CPU
> with different cache policy (e.g. sync_icache_aliases()).
>
> --
> Catalin
On Thu, May 09, 2019 at 09:50:04AM +0100, Catalin Marinas wrote:
> Hi,
>
> On Wed, May 08, 2019 at 11:45:03AM -0700, Salman Qazi wrote:
> > What is the intention behind icache_is_aliasing on big.LITTLE systems
> > where some icaches are VIPT and others are PIPT? Is it meant to be
> > conservative in some sense or should it be made per-CPU?
>
> It needs to cover the worst case scenario across all CPUs, i.e. aliasing
> VIPT if one of the CPUs has this. We can't make it per-CPU because a
> thread performing cache maintenance might be migrated to another CPU
> with different cache policy (e.g. sync_icache_aliases()).
It's slightly more subtle than that -- for broadcast maintenance the
policy of the CPU receiving the broadcast matters.
So even if all i-cache maintenance were performed on a thread pinned to
a CPU with PIPT caches, to correctly affect any VIPT i-caches in the
system it would be necessary to perform maintenance as-if the CPU
performing the maintenance had VIPT i-caches.
Thanks,
Mark.