Re: [PATCH] efidisk: pass buffers with higher alignment

[Stefan Agner]

On 2022-05-05 17:42, Vladimir 'phcoder' Serbinenko wrote:

> Le jeu. 5 mai 2022, 15:55, Stefan Agner <stefan@agner.ch> a écrit : 
> 
>> Despite the UEFI specification saying "the requirement is that the
>> start address of a buffer must be evenly divisible by IoAlign with
>> no remainder.", it seems that a higher alignment requirement is
>> neecssary on some system (e.g. a Intel NUC system with NVMe SSD).
>> That particular system has IoAlign set to 2, and sometimes returns
>> status 7 when buffers with alignment of 2 are passed. Things seem
>> to work fine with buffers aligned to 4 bytes.
>> 
>> It seems that IoAlign > 1 means 2 ^ IoAlign. There is also such a hint
>> in an example printed in the Driver Writer's Guide:
>> ScsiPassThruMode.IoAlign = 2; // Data must be alligned on 4-byte boundary
>> 
>> Pass 2 ^ IoAlign aligned buffers to make sure GRUB2 works properly on
>> all systems.
>> 
>> Note: The problem has only noticed with compressed squashfs. It seems
>> that ext4 (and presumably other file system drivers) pass buffers with
>> a higher alignment already.
>> 
>> Signed-off-by: Stefan Agner <stefan@agner.ch>
>> ---
>> grub-core/disk/efi/efidisk.c | 12 ++++++++++--
>> 1 file changed, 10 insertions(+), 2 deletions(-)
>> 
>> diff --git a/grub-core/disk/efi/efidisk.c b/grub-core/disk/efi/efidisk.c
>> index f077b5f55..0fc2f0826 100644
>> --- a/grub-core/disk/efi/efidisk.c
>> +++ b/grub-core/disk/efi/efidisk.c
>> @@ -553,8 +553,16 @@ grub_efidisk_readwrite (struct grub_disk *disk, 
>> grub_disk_addr_t sector,
>> d = disk->data;
>> bio = d->block_io;
>> 
>> -  /* Set alignment to 1 if 0 specified */
>> -  io_align = bio->media->io_align ? bio->media->io_align : 1;
>> +  /*
>> +   * If IoAlign is > 1, it means alignment by 2^IoAlign
>> +   * Note: UEFI spec claims alignment by IoAlign. But there are systems
>> +   * with IoAlign=2 which return status 7 if 2 bytes aligned buffers are
>> +   * passed.
>> +   */
>> +  if (bio->media->io_align > 1)
>> +    io_align = 1 << bio->media->io_align;
>> +  else
>> +    io_align = 1;
> 
> May be it should be >=1 rather than > 1? 
> 

No, 1 is explicitly defined to mean no alignment, hence io_align needs
to be set to 1.

--
Stefan

>> num_bytes = size << disk->log_sector_size;
>> 
>> if ((grub_addr_t) buf & (io_align - 1))
>> -- 
>> 2.36.0
>> 
>> _______________________________________________
>> Grub-devel mailing list
>> Grub-devel@gnu.org
>> https://lists.gnu.org/mailman/listinfo/grub-devel
> 
> _______________________________________________
> Grub-devel mailing list
> Grub-devel@gnu.org
> https://lists.gnu.org/mailman/listinfo/grub-devel