efi: Add efi= parameter parsing to the EFI boot stub

We need a way to customize the behaviour of the EFI boot stub, in
particular, we need a way to disable the "chunking" workaround, used
when reading files from the EFI System Partition.

One of my machines doesn't cope well when reading files in 1MB chunks to
a buffer above the 4GB mark - it appears that the "chunking" bug
workaround triggers another firmware bug. This was only discovered with
commit 4bf7111f5016 ("x86/efi: Support initrd loaded above 4G"), and
that commit is perfectly valid. The symptom I observed was a corrupt
initrd rather than any kind of crash.

efi= is now used to specify EFI parameters in two very different
execution environments, the EFI boot stub and during kernel boot.

There is also a slight performance optimization by enabling efi=nochunk,
but that's offset by the fact that you're more likely to run into
firmware issues, at least on x86. This is the rationale behind leaving
the workaround enabled by default.

Also provide some documentation for EFI_READ_CHUNK_SIZE and why we're
using the current value of 1MB.

Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Roy Franz <roy.franz@linaro.org>
Cc: Maarten Lankhorst <m.b.lankhorst@gmail.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Borislav Petkov <bp@suse.de>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 5ae8608ca9f5..67d79597d9d6 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -992,10 +992,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         Format: {"off" | "on" | "skip[mbr]"}
 
         efi=            [EFI]
-                        Format: { "old_map" }
+                        Format: { "old_map", "nochunk" }
                         old_map [X86-64]: switch to the old ioremap-based EFI
                         runtime services mapping. 32-bit still uses this one by
                         default.
+                        nochunk: disable reading files in "chunks" in the EFI
+                        boot stub, as chunking can cause problems with some
+                        firmware implementations.
 
         efi_no_storage_paranoia [EFI; X86]
                         Using this parameter you can use more than 50% of