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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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)?

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).