linux-kernel - Re: [PATCH 1/1] efi/libstub: DRAM base calculation

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Date:   Mon, 7 Sep 2020 10:30:53 +0200
From:   Heinrich Schuchardt <xypron.glpk@....de>
To:     Maxim Uvarov <maxim.uvarov@...aro.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-efi@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>
Subject: Re: [PATCH 1/1] efi/libstub: DRAM base calculation

On 07.09.20 09:00, Maxim Uvarov wrote:
> On Fri, 4 Sep 2020 at 18:50, Heinrich Schuchardt <xypron.glpk@....de> wrote:
>>
>> In the memory map the regions with the lowest addresses may be of type
>> EFI_RESERVED_TYPE. The reserved areas may be discontinuous relative to the
>> rest of the memory. So for calculating the maximum loading address for the
>> device tree and the initial ramdisk image these reserved areas should not
>> be taken into account.
>>
>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
>> ---
>>  drivers/firmware/efi/libstub/efi-stub.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/firmware/efi/libstub/efi-stub.c b/drivers/firmware/efi/libstub/efi-stub.c
>> index c2484bf75c5d..13058ac75765 100644
>> --- a/drivers/firmware/efi/libstub/efi-stub.c
>> +++ b/drivers/firmware/efi/libstub/efi-stub.c
>> @@ -106,7 +106,8 @@ static unsigned long get_dram_base(void)
>>         map.map_end = map.map + map_size;
>>
>>         for_each_efi_memory_desc_in_map(&map, md) {
>> -               if (md->attribute & EFI_MEMORY_WB) {
>> +               if (md->attribute & EFI_MEMORY_WB &&
>> +                   md->type != EFI_RESERVED_TYPE) {
>
> shouldn't the type here be CONVENTIONAL?

In 32bit ARM reserve_kernel_base() the following are considered:

* EFI_LOADER_CODE
* EFI_LOADER_DATA
* EFI_BOOT_SERVICES_CODE
* EFI_BOOT_SERVICES_DATA
* EFI_CONVENTIONAL_MEMORY

What I have not yet fully understood is why Linux on ARM 32bit tries to
put the kernel into the first 128 MiB. Cf. handle_kernel_image() in
drivers/firmware/efi/libstub/arm32-stub.c.

According to the comments this is due to some implementation detail in
the Linux zImage decompressor and not required by UEFI or the hardware:

         Verify that the DRAM base address is compatible with the ARM
         boot protocol, which determines the base of DRAM by masking
         off the low 27 bits of the address at which the zImage is
         loaded. These assumptions are made by the decompressor,
         before any memory map is available.

Best regards

Heinrich

>
>>                         if (membase > md->phys_addr)
>>                                 membase = md->phys_addr;
>>                 }
>> --
>> 2.28.0
>>