Re: [Xen-devel] [PATCH v11 07/13] x86: add multiboot2 protocol support for EFI platforms

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Thu, Jan 12, 2017 at 01:46:41PM -0600, Doug Goldstein wrote:
> On 1/12/17 1:30 PM, Daniel Kiper wrote:
> > On Thu, Jan 12, 2017 at 09:44:59AM -0600, Doug Goldstein wrote:
>
> >
> >> view there's no reason for adding MB2 support for BIOS since it provides
> >> no advantage over MB1 when booting from the BIOS. Now MB2 solves a
> >
> > From your point of view maybe it does not. However, from user point of view 
> > it may.
> > If you have support for MB2 on legacy BIOS and EFI platforms then you can 
> > boot Xen
> > on both platforms without changing anything in boot config files. Otherwise 
> > you have
> > to prepare separate configuration for different platforms.
>
> Neither Grub nor iPXE require different configs for MB1 vs MB2 so I'm
> not seeing the validity of this logic.

Hmmm... This is interesting. I do not know iPXE, however, in GRUB you must
use multiboot/module for MB1 and multiboot2/module2 for MB2. I suppose that
you have to differentiate between both of them in iPXE somehow too. Hence,
there is pretty good chance that configs for MB1 and MB2 are different.

> >> problem with booting over EFI vs MB1 so they'll be willing to take a
> >> change there. I'll also disagree that BIOS is easier than EFI since with
> >> EFI its just load the ELF into memory and set a few pointers in tags.
> >> With BIOS it requires me to build up the memory map into a MB2 structure.
> >
> > Xen uses only these tags on legacy BIOS platforms: 
> > MULTIBOOT2_TAG_TYPE_BASIC_MEMINFO
> > (well, nice to have but it can be also not provided), 
> > MULTIBOOT2_TAG_TYPE_MMAP (same
> > as MULTIBOOT2_TAG_TYPE_BASIC_MEMINFO), MULTIBOOT2_TAG_TYPE_BOOT_LOADER_NAME
> > (same as MULTIBOOT2_TAG_TYPE_BASIC_MEMINFO) ,MULTIBOOT2_TAG_TYPE_CMDLINE,
> > MULTIBOOT2_TAG_TYPE_MODULE. I do not mention MULTIBOOT2_TAG_TYPE_END which
> > is obvious. So, if you are real hardcore minimalist then you have to provide
> > MULTIBOOT2_TAG_TYPE_CMDLINE and MULTIBOOT2_TAG_TYPE_MODULE. All of them
> > are provided also on EFI. So, I do not see any reason to not provide MB2
> > for legacy BIOS. And I do not think that it is very difficult to provide
> > all optional tags mentioned above.
>
> I don't understand what you're attempting to convey here. You've listed
> out a number of tags that I mentioned in my message that I don't have to
> implement for EFI. You've basically reinforced my point that its easier
> to implement this for EFI than BIOS. MULTIBOOT2_TAG_TYPE_BASIC_MEMINFO
> and MULTIBOOT2_TAG_TYPE_MMAP are unused by Xen on EFI. It gets this info

I showed you that if you are real minimalist you can enable the same MB2 code
on legacy BIOS and EFI. I do not understand your objection against providing
MB2 in iPXE on legacy BIOS if you do not need extra code (maybe a few #ifdefs).
Though I am not going to convince you. It is your choice but I am still thinking
that it is wrong choice.

By the way, does iPXE check MULTIBOOT2_HEADER_TAG_INFORMATION_REQUEST in Xen 
header.
If it does (it should) and do not understand MULTIBOOT2_TAG_TYPE_BASIC_MEMINFO 
and
MULTIBOOT2_TAG_TYPE_MMAP then it should fail.

> from a call to GetMemoryMap(). You actually reminded me of another bug.
> Calling ExitBootServices() on Grub and letting it pass the memory info
> causes Xen to fail to load.

How come... Which GRUB version do you use? Xen clearly says that it needs
boot services (look into MB2 header). So, GRUB is not allowed to call
ExitBootServices(). If it does then it is GRUB bug.

> Andrew helped me troubleshoot this and he discovered the fix. You've got
> code:
>
> /* Store Xen image load base address in place accessible for 32-bit code. */
> mov %r15d,%esi
>
> But if any of the checks under the run_bs: label specifically:
> - /* Are EFI boot services available? */
> - /* Is EFI SystemTable address provided by boot loader? */
> - /* Is EFI ImageHandle address provided by boot loader? */
>
> Will not run the mov instruction and then fail to boot. Its only if any
> of these are false will it attempt to use the tags mentioned above as well.

OK, this is a bug and I will fix it. However, this is not related to
ExitBootServices() call in GRUB2.

Daniel

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