Re: [PATCH] efi: Reallocate memory map if ExitBootServices() fails

[Ross Lagerwall <ross.lagerwall@citrix.com>]

On 05/29/2015 10:45 AM, Jan Beulich wrote:
>>>> On 29.05.15 at 09:48, <ross.lagerwall@citrix.com> wrote:
>> --- a/xen/common/efi/boot.c
>> +++ b/xen/common/efi/boot.c
>> @@ -1053,14 +1053,14 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable)
>>               efi_arch_video_init(gop, info_size, mode_info);
>>       }
>>
>> -    efi_bs->GetMemoryMap(&efi_memmap_size, NULL, &map_key,
>> -                         &efi_mdesc_size, &mdesc_ver);
>> -    efi_memmap = efi_arch_allocate_mmap_buffer(&efi_memmap_size);
>> -    if ( !efi_memmap )
>> -        blexit(L"Unable to allocate memory for EFI memory map");
>> -
>>       for ( retry = 0; ; retry = 1 )
>>       {
>> +        efi_bs->GetMemoryMap(&efi_memmap_size, NULL, &map_key,
>> +                             &efi_mdesc_size, &mdesc_ver);
>> +        efi_memmap = efi_arch_allocate_mmap_buffer(&efi_memmap_size);
>> +        if ( !efi_memmap )
>> +            blexit(L"Unable to allocate memory for EFI memory map");
>
> You can't blexit() anymore after having called ExitBootServices() once.
> Admittedly even the PrintErrMesg() used for "error handling" a few
> lines down is on the edge of being invalid (but this is a best effort
> thing anyway).

OK, I'll convert it into a PrintErrMesg.

>
> Further you should do a second allocation only if you positively
> identified that the new size is larger than what the existing buffer
> can hold. It may be worth allocating a couple of extra entries the
> first time through anyway; perhaps that would even avoid th need
> for this workaround.

OK.

>
> Since, finally, this is only a workaround, as the spec clearly says:
> "After an Operating System calls ExitBootServices(), firmware boot
> services are no longer available and it is illegal to call any boot
> service." Even our re-invocation of GetMemoryMap() is bending
> that (and I only hesitantly agreed for it to be added).
>

The spec kinda disagrees with that:
"It is suggested that GetMemoryMap()be called immediately before calling 
ExitBootServices(). If MapKey value is incorrect, ExitBootServices() 
returns EFI_INVALID_PARAMETER and GetMemoryMap() with ExitBootServices() 
must be called again. Firmware implementation may choose to do a partial 
shutdown of the boot services during the first call to 
ExitBootServices(). EFI OS loader should not make calls to any boot 
service function other then GetMemoryMap() after the first call to 
ExitBootServices()."

I think the patch does the right thing in the regard.

-- 
Ross Lagerwall