linux-kernel - Re: [RFC 04/17] x86/asm/64: Simplify reg restore code in the standard IRET paths

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Date:   Tue, 12 Sep 2017 15:05:45 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [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@...nel.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