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

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

[Steve McIntyre]

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 <steve@einval.com>
---
 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)
+      {
+	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 */
+    {
+      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";
-- 
1.7.10.4

-- 
Steve McIntyre, Cambridge, UK.                                steve@einval.com
"You can't barbecue lettuce!" -- Ellie Crane

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 <steve@einval.com> 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 <steve@einval.com>
> ---
>  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";

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

[Steve McIntyre]

On Wed, Jan 28, 2015 at 06:42:01AM +0300, Andrei Borzenkov wrote:
>В Wed, 28 Jan 2015 00:56:30 +0000
>Steve McIntyre <steve@einval.com> пишет:
>
>> 
>> =====================================================================
>> 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 <steve@einval.com>
>> ---
>>  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

Yup.

>> +      {
>> +	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.

OK, cool.

>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.

Correct, yes.

v2 coming shortly.

-- 
Steve McIntyre, Cambridge, UK.                                steve@einval.com
Welcome my son, welcome to the machine.

[PATCH] Recognise a 64-bit Linux kernel on a 32-bit EFI firmware

[Steve McIntyre]

Some x86 systems might be capable of running a 64-bit Linux kernel but
only use a 32-bit EFI (e.g. Intel Bay Trail systems). It's useful for
grub-install to be able to recognise such systems, to set the default
x86 platform correctly.

To allow grub-install to know the size of the firmware rather than
just the size of the kernel, 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 <steve@einval.com>
---
 grub-core/osdep/linux/platform.c |   39 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/grub-core/osdep/linux/platform.c b/grub-core/osdep/linux/platform.c
index 4b9f6ef..775b6c0 100644
--- a/grub-core/osdep/linux/platform.c
+++ b/grub-core/osdep/linux/platform.c
@@ -60,6 +60,43 @@ 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) >= 3) /* 2 digits plus newline */
+      {
+	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 */
+      if (is_64_kernel ())
+	ret = 64;
+      else
+	ret = 32;
+    }
+
+  return ret;
+}
+
 const char *
 grub_install_get_default_x86_platform (void)
 { 
@@ -77,7 +114,7 @@ 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";
-- 
1.7.10.4

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

[Steve McIntyre]

On Wed, Jan 28, 2015 at 11:42:14PM +0000, Steve McIntyre wrote:
>
>v2 coming shortly.

Hi! Any comments on v2?

-- 
Steve McIntyre, Cambridge, UK.                                steve@einval.com
Google-bait:       http://www.debian.org/CD/free-linux-cd
  Debian does NOT ship free CDs. Please do NOT contact the mailing
  lists asking us to send them to you.