Re: [Xen-devel] [PATCH V4] xen/arm: domain_build: allocate lowmem for dom0 as much as possible

[Julien Grall <julien.grall@xxxxxxx>]

(CC Wei as release manager)

On 10/11/16 08:30, Peng Fan wrote:
> Hi Julien,
Hi Peng,

> On Tue, Nov 01, 2016 at 02:42:06PM +0000, Julien Grall wrote:
> > Hi Peng,
> >
> > Sorry for the late answer.
> >
> > On 23/09/2016 03:55, Peng Fan wrote:
> > > On AArch64 SoCs, some IPs may only have the capability to access
> > > 32 bits address space. The physical memory assigned for Dom0 maybe
> > > not in 4GB address space, then the IPs will not work properly.
> > > So need to allocate memory under 4GB for Dom0.
> > >
> > > There is no restriction that how much lowmem needs to be allocated for
> > > Dom0 ,so allocate lowmem as much as possible for Dom0.
> > >
> > > This patch does not affect 32-bit domain, because Variable "lowmem" is
> > > set to true at the beginning. If failed to allocate bank0 under 4GB,
> > > need to panic for 32-bit domain, because 32-bit domain requires bank0
> > > be allocated under 4GB.
> > >
> > > For 64-bit domain, set "lowmem" to false, and continue allocating
> > > memory from above 4GB.
> > >
> > > Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
> > > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > Cc: Julien Grall <julien.grall@xxxxxxx>
> > Reviewed-by: Julien Grall <julien.grall@xxxxxxx>
> >
> > I am undecided whether this should be considered as a bug fix for Xen 4.8.
> > Are you aware of any ARM64 platform we currently support requiring allocation
> > of memory below 4GB?
> I have no idea about this (:, but I think this is a bug fix. Alought current
> supported platforms works well, users may choose 4.8 to support their
> new platform which has the limitation to access 64bit address.
We are already late in the release process (rc5) for Xen 4.8. So we need 
to be careful when including a bug fix and evaluate the pros and cons.
This patch is modifying the DOM0 memory layout for all 64-bit platforms. 
So it could potentially break one of the platform we  officially support 
(see [1] for a non-exhaustive list). We don't have a test suite running 
automatically for ARM64 at the moment (it is been working on), this 
means that manual testing needs to be done. I am not aware of any 
platform, in the list we supports, having this issue so I prefer to stay 
on the safe side and defer this patch for Xen 4.9.
If a user cares about Xen 4.8 for their platforms, then they could 
request the patch to be backported in Xen 4.8 after the release and 
after extensive testing in staging.
Regards,

[1] 
https://wiki.xenproject.org/wiki/Xen_ARM_with_Virtualization_Extensions#Hardware
> Regards,
> Peng.
>
> > Regards,
> >
> > --
> > Julien Grall
--
Julien Grall

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