Re: [Xen-devel] [PATCH v3] xen: Allow a default compiled-in command line using Kconfig

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 7 Mar 2017, Julien Grall wrote:
> Hi Jan,
> 
> On 03/07/2017 02:52 PM, Jan Beulich wrote:
> > > > > On 07.03.17 at 15:41, <julien.grall@xxxxxxx> wrote:
> > > Hi Jan,
> > > 
> > > On 03/07/2017 02:18 PM, Jan Beulich wrote:
> > > > > > > On 07.03.17 at 14:48, <julien.grall@xxxxxxx> wrote:
> > > > > On 03/07/2017 12:52 PM, Jan Beulich wrote:
> > > > > > > > > On 07.03.17 at 12:21, <blackskygg@xxxxxxxxx> wrote:
> > > > > > > 2017-03-07 17:36 GMT+08:00 Jan Beulich <JBeulich@xxxxxxxx>:
> > > > > > > > > > > On 07.03.17 at 09:34, <blackskygg@xxxxxxxxx> wrote:
> > > > > > > > > +static inline char* __init extract_dom0_options(char
> > > > > > > > > *cmdline)
> > > > > > > > > +{
> > > > > > > > > +    char *kextra;
> > > > > > > > > +
> > > > > > > > > +    if ( (kextra = strstr(cmdline, " -- ")) != NULL )
> > > > > > > > > +    {
> > > > > > > > > +        *kextra = '\0';
> > > > > > > > > +        kextra += 3;
> > > > > > > > > +        while ( kextra[1] == ' ' ) kextra++;
> > > > > > > > 
> > > > > > > > The body of the while() wants to go on its own line.
> > > > > > > > 
> > > > > > > > And then - why is this Dom0 option handling done only on x86?
> > > > > > > > 
> > > > > > > 
> > > > > > > As you might have noticed, there isn't any code dealing with the
> > > > > > > dom0
> > > options
> > > > > > > in arch/arm/setup.c, and the arm version of construct_dom0()
> > > > > > > doesn't take
> > > any
> > > > > > > command line options as its parameter,
> > > > > > > so I have the reason to believe that this feature is not available
> > > > > > > under the arm architecture.
> > > > > > 
> > > > > > Looks like an omission to me - Julien, Stefano?
> > > > > 
> > > > > DOM0 and Xen command line are passed separately through either Device
> > > > > Tree or for UEFI xen configuration file (see -cfg=...).
> > > > > 
> > > > > So I am not sure to understand what would be the benefits to handle
> > > > > DOM0
> > > > > parameters after -- on Xen command line.
> > > > 
> > > > So you have no case of a boot loader which allows you to type in
> > > > extra options on just a single line? On x86 the feature had originally
> > > > been added because old grub didn't have a separate line for Dom0
> > > > options in its graphical menu. Nowadays the functionality is handy
> > > > namely when starting xen.efi from the EFI shell (where again you
> > > > obviously only have a single command line), but quite likely this may
> > > > also be of use with grub's chain loading model (which I simply don't
> > > > use very often, so I'm not finally sure on that one).
> > > 
> > > My knowledge is quite limited on boot process for x86. How do you pass
> > > the kernel/initrd/xsm blob on UEFI? Can you do it from the command line
> > > or are you using the -cfg=... and specify it in a file?
> > 
> > Only the latter.
> > 
> > > On ARM we have two way to boot Xen:
> > >   - Using UEFI bootloader with either Device-Tree or ACPI
> > >   - Using non-UEFI bootloader with Device-Tree only
> > > 
> > > In the case of UEFI bootloader, we are using the xen configuration file
> > > to describe the modules (e.g kernel, initramfs, XSM) and the both xen
> > > and DOM0 command line.
> > > 
> > > For non-UEFI bootloader, we have designed the boot protocol based on the
> > > device-tree and will allows you to specify both xen and DOM0 and all the
> > > modules (see [1]). The bootloader needs to be able to modify the
> > > device-tree (such via a shell like on U-boot) or the user needs to
> > > modify the device-tree before hand.
> > 
> > All fine, but this doesn't tell me what interactive change options a
> > user has _after_ having set up the config file (or alike), while the
> > system is booting.
> 
> Here some concrete examples. The major bootloaders on ARM today are:
>       * UEFI
>       * U-boot
>       * GRUB
> 
> I will leave UEFI aside as people will usually chainload to GRUB and then boot
> whatever they want.
> 
> In both GRUB and U-boot a user will be able to modify the command line from
> the bootloader shell.

Given that upstream GRUB doesn't yet support booting Xen on ARM (without
any additional patches), I think that the ability to completely change
the command line from the EFI shell would be useful. Besides, although
it is not mandatory, I think it would be best not to unnecessarily
diverge from x86 in terms of EFI booting.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel