Re: [Xen-devel] [PATCH] xen/arm: increase default dom0_mem to 512M

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 17 Feb 2017, Julien Grall wrote:
> Hi Stefano,
> 
> On 16/02/17 20:17, Stefano Stabellini wrote:
> > On Thu, 16 Feb 2017, Julien Grall wrote:
> > > Hi Stefano,
> > > 
> > > On 15/02/2017 23:05, Stefano Stabellini wrote:
> > > > The default dom0_mem is 128M which is not sufficient to boot a Ubuntu
> > > > based Dom0. Increase it to 512M.
> > > > 
> > > > Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > 
> > > I am not a big fan of increasing the default value. 128M is plenty enough
> > > if
> > > you use a small DOM0 (e.g buildroot or yocto) and people may rely on it
> > > because it is the default value in the documentation
> > > (see docs/misc/xen-command-line.markdown).
> > > 
> > > Also, 512M may boot Ubuntu for you but it might not be the case in all the
> > > configuration. There is no perfect default value, but I think the smaller
> > > is
> > > better. Looking at the documentation, it looks like x86 is using 128MB or
> > > 1/16
> > > of the memory (whichever is smaller).
> > > 
> > > But to be fair, I am not even sure why there is a default value, it is
> > > quite
> > > easy to specify the amount of memory used by DOM0 on the command line.
> > 
> > This is a topic particularly prone to bike-shedding :-)
> > 
> > Like you wrote, there is no perfect default value. The problem with
> > 128M is that Dom0 will fail to boot without any meaningful errors. I
> > think it makes for a poor out of the box experience: the user is trying
> > to boot Xen for the first time on her board, she hasn't customized
> > much yet, and she has to waste a couple of hours to figure out why Dom0
> > is crashing.
> > 
> > On the other end, people that are trying to use as little memory as
> > possible, they are well past the first Xen boot, and they are most
> > certainly aware of the dom0_mem parameter.
> > 
> > In other words, setting dom0_mem to 128M by default hurts first time
> > users without helping seasoned users very much.
> > 
> > Rather than having dom0_mem=128M by default, causing a dom0 crash
> > without any obvious errors, I would rather crash Xen explicitly if
> > dom0_mem is not set. That way, the user is forced to type in the
> > dom0_mem parameter and could more easily guess why dom0 is crashing.
> 
> Crashing if dom0_mem is not set would be my preferred way. At least we won't
> have a default value that is a bit random. Ubuntu uses 512MB today, but who
> knows tomorrow?

I have an idea: what if in case dom0_mem is not, we print a warning,
then wait 3 full seconds and continue with a large dom0_mem? That way,
it would be impossible to ignore, but at the same time the user could
fix the problem right away from dom0 as soon as it's booted?

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