Re: [PATCH 2/2] x86/efi: Allow invocation of arbitrary boot services

Matt Fleming
2016-09-05T13:02:57Z

On Mon, 22 Aug, at 12:01:21PM, Lukas Wunner wrote:
> We currently allow invocation of 8 boot services with efi_call_early().
> Not included are LocateHandleBuffer and LocateProtocol in particular.
> For graphics output or to retrieve PCI ROMs and Apple device properties,
> we're thus forced to use the LocateHandle + AllocatePool + LocateHandle
> combo, which is cumbersome and needs more code.
>
> The ARM folks allow invocation of the full set of boot services but are
> restricted to our 8 boot services in functions shared across arches.
>
> Thus, rather than adding just LocateHandleBuffer and LocateProtocol to
> struct efi_config, let's rework efi_call_early() to allow invocation of
> arbitrary boot services by selecting the 64 bit vs 32 bit code path in
> the macro itself.
>
> When compiling for 32 bit or for 64 bit without mixed mode, the unused
> code path is optimized away and the binary code is the same as before.
> But on 64 bit with mixed mode enabled, this commit adds one compare
> instruction to each invocation of a boot service and, depending on the
> code path selected, two jump instructions. (Most of the time gcc
> arranges the jumps in the 32 bit code path.) The result is a minuscule
> performance penalty and the binary code becomes slightly larger and more
> difficult to read when disassembled. This isn't a hot path, so these
> drawbacks are arguably outweighed by the attainable simplification of
> the C code. We have some overhead anyway for thunking or conversion
> between calling conventions.
>
> The 8 boot services can consequently be removed from struct efi_config.
>
> No functional change intended (for now).
>
> Example -- invocation of free_pool before (64 bit code path):
> 0x2d4 movq %ds:efi_early, %rdx ; efi_early
> 0x2db movq %ss:arg_0-0x20(%rsp), %rsi
> 0x2e0 xorl %eax, %eax
> 0x2e2 movq %ds:0x28(%rdx), %rdi ; efi_early->free_pool
> 0x2e6 callq *%ds:0x58(%rdx) ; efi_early->call()
>
> Example -- invocation of free_pool after (64 / 32 bit mixed code path):
> 0x0dc movq %ds:efi_early, %rax ; efi_early
> 0x0e3 cmpb $0, %ds:0x28(%rax) ; !efi_early->is64 ?
> 0x0e7 movq %ds:0x20(%rax), %rdx ; efi_early->call()
> 0x0eb movq %ds:0x10(%rax), %rax ; efi_early->boot_services
> 0x0ef je $0x150
> 0x0f1 movq %ds:0x48(%rax), %rdi ; free_pool (64 bit)
> 0x0f5 xorl %eax, %eax
> 0x0f7 callq *%rdx
> ...
> 0x150 movl %ds:0x30(%rax), %edi ; free_pool (32 bit)
> 0x153 jmp $0x0f5
>
> Size of eboot.o text section:
> CONFIG_X86_32: 6464 before, 6318 after
> CONFIG_X86_64 && !CONFIG_EFI_MIXED: 7670 before, 7573 after
> CONFIG_X86_64 && CONFIG_EFI_MIXED: 7670 before, 8319 after
>
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> Cc: Matt Fleming <matt@codeblueprint.co.uk>
> ---
> arch/x86/boot/compressed/eboot.c | 13 +------------
> arch/x86/boot/compressed/head_32.S | 6 +++---
> arch/x86/boot/compressed/head_64.S | 8 ++++----
> arch/x86/include/asm/efi.h | 15 ++++++---------
> 4 files changed, 14 insertions(+), 28 deletions(-)

Nice, I like this. Applied pending the update to the first patch.