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

Rob Clark
2017-07-25T12:28:12Z

On Tue, Jul 25, 2017 at 8:16 AM, Alexander Graf <agraf@suse.de> wrote:
>
>
> On 25.07.17 14:08, Rob Clark wrote:
>>
>> On Tue, Jul 25, 2017 at 7:17 AM, Alexander Graf <agraf@suse.de> wrote:
>>>
>>>
>>>
>>> On 25.07.17 13:10, Rob Clark wrote:
>>>>
>>>>
>>>> On Tue, Jul 25, 2017 at 4:10 AM, Alexander Graf <agraf@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@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.
>>>
>>
>> ugg.. gd is a gd pita ;-)
>>
>> how many places do we have callbacks into UEFI context like this? If
>> just one or two maybe we can suppress them while in u-boot context and
>> handle in efi_exit_func() when entry_count drops to zero?
>
>
> What do you mean by suppressing? We need to call those helpers
> synchronously. And yes, we can probably hand massage the refcount on the
> ones that we remember, but I'm just afraid that the whole thing will be so
> complicated eventually that nobody understands what's going on.

Maybe suppress isn't the right word.. I was thinking of delaying the
callback until EFI_EXIT() that transitions back to the UEFI world. So
from the PoV of the UEFI world, it is still synchronous.

I haven't looked at the efi_event stuff until just now, but from a
30sec look, maybe efi_signal_event() could just put the event on a
list of signalled events and not immediately call
event->notify_function(), and handle the calling of notify_function()
in the last EFI_EXIT()??

> I personally think having _ext functions in anything exposed to UEFI
> payloads makes more sense.
>

having 2x all the interfaces for file related protocols would be
unfortunate. (Also currently they can stay static in efi_file.c and
just be called via same efi_file_handle fxn ptrs that the UEFI world
would be using, which is kinda nice.)