Re: [Xen-devel] [MULTIBOOT2 DOC PATCH v2 05/11] multiboot2: Add description of support for EFI boot services

[Daniel Kiper <dkiper@xxxxxxxxxxxx>]

Hi Toomas,

Thank you for comments.

On Thu, Nov 24, 2016 at 11:47:48PM +0200, Toomas Soome wrote:
> There is still the same confusion as with entry address tag 7; the diagram has

I suppose that you thought about tag type 3.

> u_virt, yet the text does claim it is physical address. Sure, it is assumed 
> the
> identity mapping, but still those names are creating confusion.

If yes then I agree that u_virt should be changed to u_phys. Though maybe in
longterm we should define tags for every architecture separately with types
specific for a given architecture. Then we should avoid most of such confusions.
I hope...

> Moreover, the EFI32 and EFI64 have different address sizes (32 versus 64 bit),
> yet there is the same type: u_virt - so it hints the upper part of the 64bit
> address is ignored, but its not really clear from the text???

It is clear that it should be u_phys or u32 for EFI i386. Though I am not sure
about EFI amd64 right now. Potentially it could be u64. However, I assume (at
least right now; and current GRUB implementation works in that way) that the 
boot
loader cannot put any part of an image higher than 4 GiB - 1 (well, I agree that
it should be mentioned in spec). Even on EFI amd64 platform. This is because 
still
some of the image boot code can be executed in 32-bit mode. We have such 
situation
in Xen. So, from this point of view 32-bit entry_addr in tag type 9 is 
sufficient.
Though I can agree that we should anticipate support for full 64-bit images.
I think that it is possible (but not implemented yet). We should add to spec tag
which says: yes, I am 64-bit image. Then the image may provide relevant 64-bit
equivalent tags (defined properly in spec). So, in this case we should have in 
spec
another EFI amd64 tag which has 64-bit entry_addr. This way various boot code 
types
(32-bit and 64-bit) may coexist without any issue in one image and boot loader 
may
choose supported one. Does it make sense?

Daniel

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