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Date:   Thu, 29 Nov 2018 17:49:14 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>, julia@ni.com, jeyu@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
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On Thu, Nov 29, 2018 at 10:33:42AM -0600, Josh Poimboeuf wrote:
> > can't we 'fix' that again? The alternative is moving that IRET-frame and
> > fixing everything up, which is going to be fragile, ugly and such
> > things more.

> This seems to work...

That's almost too easy... nice!

> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index ce25d84023c0..184523447d35 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -876,7 +876,7 @@ apicinterrupt IRQ_WORK_VECTOR			irq_work_interrupt		smp_irq_work_interrupt
>   * @paranoid == 2 is special: the stub will never switch stacks.  This is for
>   * #DF: if the thread stack is somehow unusable, we'll still get a useful OOPS.
>   */
> -.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
> +.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1 create_gap=0
>  ENTRY(\sym)
>  	UNWIND_HINT_IRET_REGS offset=\has_error_code*8
>  
> @@ -891,6 +891,12 @@ ENTRY(\sym)
>  	pushq	$-1				/* ORIG_RAX: no syscall to restart */
>  	.endif
>  
> +	.if \create_gap == 1
> +	.rept 6
> +	pushq 5*8(%rsp)
> +	.endr
> +	.endif
> +
>  	.if \paranoid == 1
>  	testb	$3, CS-ORIG_RAX(%rsp)		/* If coming from userspace, switch stacks */
>  	jnz	.Lfrom_usermode_switch_stack_\@
> @@ -1126,7 +1132,7 @@ apicinterrupt3 HYPERV_STIMER0_VECTOR \
>  #endif /* CONFIG_HYPERV */
>  
>  idtentry debug			do_debug		has_error_code=0	paranoid=1 shift_ist=DEBUG_STACK
> -idtentry int3			do_int3			has_error_code=0
> +idtentry int3			do_int3			has_error_code=0	create_gap=1
>  idtentry stack_segment		do_stack_segment	has_error_code=1
>  
>  #ifdef CONFIG_XEN_PV