[U-Boot] [PATCH] efi_loader: allow efi_loader code to call EFI interfaces

[Alexander Graf]

On 25.07.17 13:10, Rob Clark wrote:
> On Tue, Jul 25, 2017 at 4:10 AM, Alexander Graf <agraf at suse.de> wrote:
>>
>>
>> On 25.07.17 01:47, Rob Clark wrote:
>>>
>>> To implement efi_load_image() for the case of loading an image from a
>>> device path rather than image already loaded into source_buffer, it is
>>> convenient to be able to re-use simple-file-system and efi-file
>>> interfaces.  But these are already using EFI_ENTER()/EFI_EXIT().  Allow
>>> entry into EFI interfaces to be recursive, since this greatly simplifies
>>> things.
>>>
>>> (And hypothetically this would be needed anyways to allow grub to call
>>> into interfaces installed by something outside of grub.)
>>>
>>> Signed-off-by: Rob Clark <robdclark at gmail.com>
>>
>>
>> So there are 2 ways to do this:
>>
>>    1) Keep a refcount, only transition when passing the 0 level
>>    2) Explicitly split functions with ENTRY/EXIT from their core functions
>>
>> So far we used approach 2, splitting functions that are used by both
>> internal and external code into _ext (for externally called) and normal
>> functions. You can see this pattern quite a few times throughout efi_loader.
>>
>> I definitely did try the refcount approach back when I decided for approach
>> 2 and it failed on me in some case, but I can't remember where.
>>
>> Either way, we should definitely be consistent. Either we always use
>> refcounting or we shouldn't bother with it. So if you can make a version
>> work where all _ext variants disappear and we're magically reentrant, I'll
>> be happy to take that. I'm fairly sure it'll break somewhere though :).
>>
> 
> for load_image via file-path, we end up needing a *bunch* of functions
> normally called via EFI.. so it is going to be a lot more _ext
> variants.

Yes, but imagine a case like this:

open_protocol
   -> open()
     -> random_junk()
       -> efi_timer_check()
         -> timer notifier
           -> console print()

Here the refcounting will simply fail on us, as the timer notifier needs 
to be run in UEFI context while efi_timer_check() as well as console 
print() need to be run in U-Boot context.

And if we start to mix and mesh the 2 approaches, I can promise you that 
2 weeks down some corner case nobody thought of falls apart because 
people don't manage to fully grasp the code flow anymore.


Alex