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SubjectRe: [RFC 04/17] x86/asm/64: Simplify reg restore code in the standard IRET paths
On Wed, Sep 06, 2017 at 02:36:49PM -0700, Andy Lutomirski wrote:
> The old code restored all the registers with movq instead of pop.
> In theory, this was done because some CPUs have higher movq
> throughput, but any gain there would be tiny and is almost certainly
> outweighed by the higher text size.
>
> This saves 96 bytes of text.
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
> arch/x86/entry/calling.h | 9 +++++++++
> arch/x86/entry/entry_64.S | 28 ++++++++++++++++++++++------
> 2 files changed, 31 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 05ed3d393da7..0a2c73fe2cfc 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -147,6 +147,15 @@ For 32-bit we have the following conventions - kernel is built with
> movq 5*8+\offset(%rsp), %rbx
> .endm
>
> + .macro POP_EXTRA_REGS
> + popq %r15
> + popq %r14
> + popq %r13
> + popq %r12
> + popq %rbp
> + popq %rbx
> + .endm
> +
> .macro RESTORE_C_REGS_HELPER rstor_rax=1, rstor_rcx=1, rstor_r11=1, rstor_r8910=1, rstor_rdx=1
> .if \rstor_r11
> movq 6*8(%rsp), %r11
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 2cd01ed9cd59..7f1a83b17b4a 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -521,9 +521,17 @@ GLOBAL(retint_user)
>
> GLOBAL(swapgs_restore_regs_and_return_to_usermode)
> SWAPGS
> - RESTORE_EXTRA_REGS
> - RESTORE_C_REGS
> - REMOVE_PT_GPREGS_FROM_STACK 8
> + POP_EXTRA_REGS
> + popq %r11
> + popq %r10
> + popq %r9
> + popq %r8
> + popq %rax
> + popq %rcx
> + popq %rdx
> + popq %rsi
> + popq %rdi
> + addq $8, %rsp
> INTERRUPT_RETURN
>
>
> @@ -546,9 +554,17 @@ retint_kernel:
> TRACE_IRQS_IRETQ
>
> GLOBAL(restore_regs_and_return_to_kernel)
> - RESTORE_EXTRA_REGS
> - RESTORE_C_REGS
> - REMOVE_PT_GPREGS_FROM_STACK 8
> + POP_EXTRA_REGS
> + popq %r11
> + popq %r10
> + popq %r9
> + popq %r8
> + popq %rax
> + popq %rcx
> + popq %rdx
> + popq %rsi
> + popq %rdi
> + addq $8, %rsp
> INTERRUPT_RETURN

Any reason why these aren't in a POP_C_REGS macro? I think that would
make it easier to verify correctness when reading the code.

--
Josh

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