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

[Julien Grall <julien.grall@xxxxxxx>]

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?
In any case, any change on the command line needs to be documented in 
docs/misc/xen-command-line.markdown.
Cheers,

--
Julien Grall

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