[PATCH v2 04/10] arm64/efi: invert UEFI memory region reservation logic

[Ard Biesheuvel]

On 28 October 2014 17:47, Mark Rutland <mark.rutland at arm.com> wrote:
> On Tue, Oct 28, 2014 at 04:18:37PM +0000, Ard Biesheuvel wrote:
>> Instead of reserving the memory regions based on which types we know
>> need to be reserved, consider only regions of the following types as
>> free for general use by the OS:
>>
>> EFI_LOADER_CODE
>> EFI_LOADER_DATA
>> EFI_BOOT_SERVICES_CODE
>> EFI_BOOT_SERVICES_DATA
>> EFI_CONVENTIONAL_MEMORY
>>
>> Note that this also fixes a problem with the original code, which would
>> misidentify a EFI_RUNTIME_SERVICES_DATA region as not reserved if it
>> does not have the EFI_MEMORY_RUNTIME attribute set. However, it is
>> perfectly legal for the firmware not to request a virtual mapping for
>> EFI_RUNTIME_SERVICES_DATA regions that contain configuration tables, in
>> which case the EFI_MEMORY_RUNTIME attribute would not be set.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>  arch/arm64/kernel/efi.c | 20 ++++++++++----------
>>  1 file changed, 10 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
>> index 95c49ebc660d..2e829148fb36 100644
>> --- a/arch/arm64/kernel/efi.c
>> +++ b/arch/arm64/kernel/efi.c
>> @@ -125,17 +125,17 @@ out:
>>   */
>>  static __init int is_reserve_region(efi_memory_desc_t *md)
>>  {
>> -     if (!is_normal_ram(md))
>> +     switch (md->type) {
>> +     case EFI_LOADER_CODE:
>> +     case EFI_LOADER_DATA:
>> +     case EFI_BOOT_SERVICES_CODE:
>> +     case EFI_BOOT_SERVICES_DATA:
>> +     case EFI_CONVENTIONAL_MEMORY:
>>               return 0;
>> -
>> -     if (md->attribute & EFI_MEMORY_RUNTIME)
>> -             return 1;
>> -
>> -     if (md->type == EFI_ACPI_RECLAIM_MEMORY ||
>> -         md->type == EFI_RESERVED_TYPE)
>> -             return 1;
>> -
>> -     return 0;
>> +     default:
>> +             break;
>> +     }
>> +     return is_normal_ram(md);
>
> Just to check: did we figure out if UnusableMemory was allowed to have
> EFI_MEMORY_WB attributes? If it isn't, this looks fine to me.
>
> If it is, then we will need to remove that memory (rather than reserving
> it) to prevent speculative accesses.
>

The spec does not mention at all how EfiUnusableMemory should be used,
and I would assume any such regions to have the EFI_MEMORY_WB
attribute set, as it is carved out of the normal system RAM, and the
way Tianocore/EDK2 implements it at least, all those attributes
(including the write-protect/execute-protect ones) are copied straight
from the underlying regions and never set to reflect the nature of the
actual contents.

However, for 3.20 I intend to propose another change to this code, so
that only non-reserved, usable memory gets memblock_add()'ed in the
first place, and I suppose this should cover your concern as well. The
reason for doing that is to allow tools like dmidecode and lshw access
to the SMBIOS and other tables through /dev/mem, which is currently
disallowed when STRICT_DEVMEM is set.

-- 
Ard.