Re: [PATCH-4.16 v2] xen/efi: Fix Grub2 boot on arm64

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.11.2021 16:33, Stefano Stabellini wrote:
> On Fri, 5 Nov 2021, Jan Beulich wrote:
>> On 04.11.2021 22:50, Stefano Stabellini wrote:
>>> On Thu, 4 Nov 2021, Luca Fancellu wrote:
>>>>> On 4 Nov 2021, at 21:35, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
>>>>> wrote:
>>>>> On Thu, 4 Nov 2021, Luca Fancellu wrote:
>>>>>>> On 4 Nov 2021, at 20:56, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
>>>>>>> wrote:
>>>>>>> @@ -851,10 +853,14 @@ static int __init 
>>>>>>> handle_dom0less_domain_node(EFI_FILE_HANDLE dir_handle,
>>>>>>> * dom0 and domU guests to be loaded.
>>>>>>> * Returns the number of multiboot modules found or a negative number 
>>>>>>> for error.
>>>>>>> */
>>>>>>> -static int __init efi_check_dt_boot(EFI_FILE_HANDLE dir_handle)
>>>>>>> +static int __init efi_check_dt_boot(EFI_LOADED_IMAGE *loaded_image)
>>>>>>> {
>>>>>>>    int chosen, node, addr_len, size_len;
>>>>>>>    unsigned int i = 0, modules_found = 0;
>>>>>>> +    EFI_FILE_HANDLE dir_handle;
>>>>>>> +    CHAR16 *file_name;
>>>>>>> +
>>>>>>> +    dir_handle = get_parent_handle(loaded_image, &file_name);
>>>>>>
>>>>>> We can’t use get_parent_handle here because we will end up with the same 
>>>>>> problem,
>>>>>> we would need to use the filesystem if and only if we need to use it, 
>>>>>
>>>>> Understood, but it would work the same way as this version of the patch:
>>>>> if we end up calling read_file with dir_handle == NULL, then read_file
>>>>> would do:
>>>>>
>>>>>  blexit(L"Error: No access to the filesystem");
>>>>>
>>>>> If we don't end up calling read_file, then everything works even if
>>>>> dir_handle == NULL. Right?
>>>>
>>>> Oh yes sorry my bad Stefano! Having this version of the patch, it will 
>>>> work.
>>>>
>>>> My understanding was instead that the Jan suggestion is to revert the place
>>>> of call of get_parent_handle (like in your code diff), but also to remove 
>>>> the
>>>> changes to get_parent_handle and read_file.
>>>> I guess Jan will specify his preference, but if he meant the last one, then
>>>> the only way I see...
>>>
>>> I think we should keep the changes to get_parent_handle and read_file,
>>> otherwise it will make it awkward, and those changes are good in their
>>> own right anyway.
>>
>> As a maintainer of this code I'm afraid I have to say that I disagree.
>> These changes were actually part of the reason why I went and looked
>> how things could (and imo ought to be) done differently.
> 
> You know this code and EFI booting better than me -- aren't you
> concerned about Xen calling get_parent_handle / dir_handle->Close so
> many times (by allocate_module_file)?

I'm not overly concerned there; my primary concern is for it to get called
without need in the first place.

> My main concern is performance and resource utilization. With v3 of the
> patch get_parent_handle will get called for every module to be loaded.
> With dom0less, it could easily get called 10 times or more. Taking a
> look at get_parent_handle, the Xen side doesn't seem small and I have
> no idea how the EDK2 side looks. I am just worried that it would
> actually have an impact on boot times (also depending on the bootloader
> implementation).

The biggest part of the function deals with determining the "residual" of
the file name. That part looks to be of no interest at all to
allocate_module_file() (whether that's actually correct I can't tell). I
don't see why this couldn't be made conditional (e.g. by passing in NULL
for "leaf").

Jan