Re: [RFC 2/2] efi/memattr: add efi_mem_attr_table as a reserved region in 820_table_firmware

[Usama Arif <usamaarif642@gmail.com>]

On 10/01/2025 14:31, Usama Arif wrote:
> 
> 
> On 10/01/2025 07:32, Ard Biesheuvel wrote:
>> On Thu, 9 Jan 2025 at 17:32, Usama Arif <usamaarif642@gmail.com> wrote:
>>>
>>>
>>>
>>> On 09/01/2025 16:15, Ard Biesheuvel wrote:
> 
>>> I think in the end whoevers' responsibility it is, the easiest path forward
>>> seems to be in kernel? (and not firmware or libstub)
>>>
>>
>> Agreed. But as I pointed out in the other thread, the memory
>> attributes table only augments the memory map with permission
>> information, and can be disregarded, and given how badly we mangle the
>> memory map on x86, maybe this is the right choice here.
>>
>>>>
>>>>> The next ideal place would be in libstub. However, it looks like
>>>>> InstallMemoryAttributesTable [2] is not available as a boot service
>>>>> call option [3], [4], and install_configuration_table does not
>>>>> seem to work as a valid substitute.
>>>>>
>>>>
>>>> To do what, exactly?
>>>>
>>>
>>> To change the memory type from System RAM to either reserved or
>>> something more appropriate, i.e. any type that is not touched by
>>> kexec or any other userspace.
>>>
>>> Basically the example code I attached at the end of the cover letter in
>>> https://lore.kernel.org/all/20250108215957.3437660-1-usamaarif642@gmail.com/
>>> It could be EFI_ACPI_RECLAIM_MEMORY or EFI_RESERVED_TYPE, both of which aren't
>>> touched by kexec.
>>>
>>
>> This is a kexec problem (on x86 only) so let's fix it there.
> 
> 
> I don't believe we can accurately tell if we are booting from a cold boot or kexec.
> There is bootloader_type available for x86, but not sure if we should rely on
> that. I think a way forward would be to move it behind a Kconfig option, something like
> below, which defaults to n for x86. Anyone who needs it can enable it. What do you think?
> 

Or we can do something like below?

diff --git a/drivers/firmware/efi/memattr.c b/drivers/firmware/efi/memattr.c
index d131781e2d7b..4add694b18d0 100644
--- a/drivers/firmware/efi/memattr.c
+++ b/drivers/firmware/efi/memattr.c
@@ -24,6 +24,15 @@ int __init efi_memattr_init(void)
        efi_memory_attributes_table_t *tbl;
        unsigned long size;
 
+#ifdef CONFIG_X86_64
+       /*
+        * On x86_64, do not initialize memory attributes table
+        * if booting from kexec
+        */
+       if (bootloader_type >> 4 == 0xd)
+               return 0;
+#endif
+
        if (efi_mem_attr_table == EFI_INVALID_TABLE_ADDR)
                return 0;