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[7/8] Documentation/x86/boot: Clarify segment requirements for EFI handover

Message ID 20200130200440.1796058-8-nivedita@alum.mit.edu
State New
Headers show
Series x86/efi,boot: GDT handling cleanup/fixes | expand

Commit Message

Arvind Sankar Jan. 30, 2020, 8:04 p.m. UTC 
The 32-bit EFI handover entry point requires segments to be setup in the
same way as for the regular 32-bit boot.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
 Documentation/x86/boot.rst | 6 ++++++
 1 file changed, 6 insertions(+)
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Patch

diff --git a/Documentation/x86/boot.rst b/Documentation/x86/boot.rst
index c9c201596c3e..3e13b7d57271 100644
--- a/Documentation/x86/boot.rst
+++ b/Documentation/x86/boot.rst
@@ -1412,6 +1412,12 @@  from the boot media and jump to the EFI handover protocol entry point
 which is hdr->handover_offset bytes from the beginning of
 startup_{32,64}.
 
+For the 32-bit handover entry point, the GDT and segments must be setup as for
+the 32-bit boot protocol, i.e. a GDT must be loaded with the descriptors for
+selectors __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
+segment; __BOOT_CS must have execute/read permission, and __BOOT_DS must have
+read/write permission; CS must be __BOOT_CS and DS, ES, SS must be __BOOT_DS.
+
 The function prototype for the handover entry point looks like this::
 
     efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)