Re: [PATCH] Add support for running a 64-bit Linux kernel on a 32-bit EFI

[Andrei Borzenkov]

В Wed, 28 Jan 2015 00:56:30 +0000
Steve McIntyre <address@hidden> пишет:

> Hi folks,
> 
> I've been working in Debian on adding support for amd64 platforms
> which are shipped with 32-bit UEFI firmware, such as the Asus X205TA
> and other Bay Trail machines. As part of that, I've had a patch
> accepted by the Linux EFI maintainers [1] to expose the size of the
> underlying firmware to userland via a new sysfs file. It should make
> mainline Linux very soon.
> 
> Using that patch as a basis, I've patched grub to use the new sysfs
> interface to work out the correct default x86 platform to match the
> firmware. Colin's happy with the patch and has just accepted it for
> Debian's grub package for now; I promised to submit it upstream for
> review, so here it is. :-)
> 
> Feedback welcome please!
> 
> [1] http://article.gmane.org/gmane.linux.kernel.efi/5229
> 
> =====================================================================
> Some platforms might be capable of running a 64-bit Linux kernel but
> only use a 32-bit EFI.  To support such systems, it is necessary to
> work out the size of the firmware rather than just the size of the
> kernel. To enable that, there is now an extra EFI sysfs file to
> describe the underlying firmware.  Read that if possible, otherwise
> fall back to the kernel type as before.
> 
> Signed-off-by: Steve McIntyre <address@hidden>
> ---
>  grub-core/osdep/linux/platform.c |   38 
> +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/grub-core/osdep/linux/platform.c 
> b/grub-core/osdep/linux/platform.c
> index 4b9f6ef..5668ae5 100644
> --- a/grub-core/osdep/linux/platform.c
> +++ b/grub-core/osdep/linux/platform.c
> @@ -60,6 +60,42 @@ is_64_kernel (void)
>    return strcmp (un.machine, "x86_64") == 0;
>  }
>  
> +static int
> +read_platform_size (void)
> +{
> +  FILE *fp;
> +  char *buf = NULL;
> +  size_t len = 0;
> +  int ret = 0;
> +
> +  /* Newer kernels can tell us directly about the size of the
> +   * underlying firmware - let's see if that interface is there. */
> +  fp = grub_util_fopen ("/sys/firmware/efi/fw_platform_size", "r");
> +  if (fp != NULL)
> +  {
> +    if (getline (&buf, &len, fp) > 0)

size >= 2

> +      {
> +     if (strncmp (buf, "32", 2) == 0)
> +       ret = 32;
> +     else if (strncmp (buf, "64", 2) == 0)
> +       ret = 64;
> +      }
> +    free (buf);
> +    fclose (fp);
> +  }
> +
> +  if (ret == 0)
> +    /* Unrecognised - fall back to matching the kernel size instead */
> +    {

I usually prefer comments inside braces and indented accordingly.

Could you send it suitable for git am? Mentioning platforms in commit
message would be useful for reference. BTW, re subject - this is about
installing 32 bit EFI grub on 64 bit Linux, not running, right? It
already should be able to run kernel if installed appropriately
manually.

> +      if (is_64_kernel ())
> +     ret = 64;
> +      else
> +     ret = 32;
> +    }
> +
> +  return ret;
> +}
> +
>  const char *
>  grub_install_get_default_x86_platform (void)
>  { 
> @@ -77,7 +113,7 @@ grub_install_get_default_x86_platform (void)
>    if (is_not_empty_directory ("/sys/firmware/efi"))
>      {
>        grub_util_info ("...found");
> -      if (is_64_kernel ())
> +      if (read_platform_size() == 64)
>       return "x86_64-efi";
>        else
>       return "i386-efi";