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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 10 Jun 2019, Julien Grall wrote:
> 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. [...]"

That's great thank you. With that, add my reviewed-by.


> > > > 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?

No, thankfully I don't :-)

I was worried that some big.LITTLE SoCs might be built like that so. (I
don't have any big.LITTLE machines here to confirm/deny.) It is good
that we don't have to worry about it.

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