Re: [Xen-devel] [PATCH 13/20] xen/arm32: mm: Avoid to zero and clean cache for CPU0 domheap

[Julien Grall <julien.grall@xxxxxxx>]

On 03/05/2019 16:57, Andrii Anisov wrote:
> Hello Julien,
Hi,

> On 22.04.19 19:49, 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 CPU0 domheap is unnecessary.
> >
> > Furthermore, CPU0 page-tables are part of Xen binary and will already be
> > zeroed beforehand.
> IMO it is a bit confusing.
> As I understand, `cpu0_dommap` resides in BSS which is not a part of the binary 
> unlike initialized data. Yet it is unconditionally cleared during the boot on 
> ARM32.
In C, uninitialized global variable will be zero by default. It is a bit of 
waste to allocate space in the binary for them. So the compiler will commonly 
put them in a section BSS that are going to be zeroed when at launch.
On Arm32, this is always done in CPU0 at early boot. For Arm64, UEFI will do it 
for us, so we don't want to do it when using UEFI as we may override global
The reason I chose to say "will always be zeroed beforehand" than specifically 
mention "BSS" is I wasn't entirely convinced the compiler will always put in BSS.
Cheers,

--
Julien Grall

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