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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Nov 10, 2016 at 01:01:38PM +0000, Julien Grall wrote:
> (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.
> 

I agree with your reasoning.

Wei.

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