Re: [Xen-devel] [PATCH MM-PART2 RESEND v2 14/19] xen/arm32: mm: Avoid cleaning the cache for secondary CPUs page-tables

[Julien Grall <julien.grall@xxxxxxx>]

Hi Steano,

On 6/10/19 9:28 PM, Stefano Stabellini wrote:
> On Wed, 5 Jun 2019, Julien Grall wrote:
> > On 05/06/2019 00:11, Stefano Stabellini wrote:
> > > On Tue, 14 May 2019, Julien Grall wrote:
> > > > The page-table walker is configured to use the same shareability and
> > > > cacheability as the access performed when updating the page-tables. This
> > > > means cleaning the cache for secondary CPUs runtime page-tables is
> > > > unnecessary.
> > > All right. Is there an explicit configuration for the shareability and
> > > cacheability used by the page-table walker or is it specified as such in
> > > the Arm Arm?
> > See the configuration of TCR_EL2, I can mention it.
>   
> That would be nice. I double-checked and it is as you wrote.
Sure. How about:

"The page-table walker is configured by TCR_EL2 to use shareability and 
cacheability as the access performed when updating the page-tables. [...]"
> > > Also, isn't it possible that CPUs on a different cluster
> > > (big.LITTLE) would have issues with this if the cache could be split
> > > between the two clusters?
> > I don't understand this... Cache should be coherent when a CPU leaves EL3.
> > But we already share some bits of the page tables between the processor (see
> > create_xen_page_tables). So I don't see where there is a possible problem
> > here.
> If the cache is always coherent across the clusters and the
> pagetable-walkers of different clusters, then this is fine.
Xen (and Linux) built on the assumption that all the CPUs (and 
page-table walker) are in the same shareable domain (i.e 
innershareable). If you have a platform where it is not the case, then 
Xen is going to be badly broken.
This is also inline with the expectatio  from the Arm Arm (B2-123 in DDI 
0487D.a):
"The Inner Shareable domain is expected to be the set of PEs controlled 
by a single hypervisor or operating system."
Do you have a case where Xen needs to run on PEs in different domains?

Cheers,

--
Julien Grall

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