linux-kernel - Re: [PATCHv4 11/19] arm64: don't reload GPRs after apply

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Message-ID: <20180709142159.tsxgwhjm3qziymym@lakrids.cambridge.arm.com>
Date:   Mon, 9 Jul 2018 15:21:59 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, dave.martin@....com, hch@...radead.org,
        james.morse@....com, linux@...inikbrodowski.net,
        linux-fsdevel@...r.kernel.org, marc.zyngier@....com,
        viro@...iv.linux.org.uk
Subject: Re: [PATCHv4 11/19] arm64: don't reload GPRs after apply_ssbd

On Fri, Jul 06, 2018 at 05:38:45PM +0100, Will Deacon wrote:
> On Mon, Jul 02, 2018 at 12:04:07PM +0100, Mark Rutland wrote:
> > Now that all of the syscall logic works on the saved pt_regs, apply_ssbd
> > can safely corrupt x0-x3 in the entry paths, and we no longer need to
> > restore them. So let's remove the logic doing so.
> > 
> > With that logic gone, we can fold the branch target into the macro, so
> > that callers need not deal with this. GAS provides \@, which provides a
> > unique value per macro invocation, which we can use to create a unique
> > label.
> > 
> > Signed-off-by: Mark Rutland <mark.rutland@....com>
> > Acked-by: Marc Zyngier <marc.zyngier@....com>
> > Acked-by: Catalin Marinas <catalin.marinas@....com>
> > Cc: Will Deacon <will.deacon@....com>
> > ---
> >  arch/arm64/kernel/entry.S | 20 +++++++-------------
> >  1 file changed, 7 insertions(+), 13 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > index c41b84d06644..728bc7cc5bbb 100644
> > --- a/arch/arm64/kernel/entry.S
> > +++ b/arch/arm64/kernel/entry.S
> > @@ -130,20 +130,21 @@ alternative_else_nop_endif
> >  
> >  	// This macro corrupts x0-x3. It is the caller's duty
> >  	// to save/restore them if required.
> > -	.macro	apply_ssbd, state, targ, tmp1, tmp2
> > +	.macro	apply_ssbd, state, tmp1, tmp2
> >  #ifdef CONFIG_ARM64_SSBD
> >  alternative_cb	arm64_enable_wa2_handling
> > -	b	\targ
> > +	b	skip_apply_ssbd\@
> >  alternative_cb_end
> >  	ldr_this_cpu	\tmp2, arm64_ssbd_callback_required, \tmp1
> > -	cbz	\tmp2, \targ
> > +	cbz	\tmp2, skip_apply_ssbd\@
> >  	ldr	\tmp2, [tsk, #TSK_TI_FLAGS]
> > -	tbnz	\tmp2, #TIF_SSBD, \targ
> > +	tbnz	\tmp2, #TIF_SSBD, skip_apply_ssbd\@
> 
> Talking to Dave, he makes a good point that this is pretty fragile if a
> macro expansion within the macro itself uses \@, since this would result
> in an unexpected label update and everything would go wrong.

I don't believe that's a problem; \@ is handled as-if it's a named
argument to the macro, and is not incremented within the scope of a
single macro expansion.

e.g. if I assemble:

----
.macro nop_macro
.endm

.macro a n
        a_pre_\n\()_\@:
        .quad   0
        a_post_\n\()_\@:
        .quad   0
.endm

.macro b n
        b_pre_\n\()_\@:
        .quad   0

        a \n

        b_post_\n\()_\@:
        .quad   0
.endm

.macro c n
        c_pre_\n\()_\@:
        .quad   0

        b \n

        c_post_\n\()_\@:
        .quad   0
.endm

.data

        a 0

        b 1

        c 2
----

... then objdump -D gives me:

----
Disassembly of section .data:

0000000000000000 <a_pre_0_0>:
        ...

0000000000000008 <a_post_0_0>:
        ...

0000000000000010 <b_pre_1_1>:
        ...

0000000000000018 <a_pre_1_2>:
        ...

0000000000000020 <a_post_1_2>:
        ...

0000000000000028 <b_post_1_1>:
        ...

0000000000000030 <c_pre_2_3>:
        ...

0000000000000038 <b_pre_2_4>:
        ...

0000000000000040 <a_pre_2_5>:
        ...

0000000000000048 <a_post_2_5>:
        ...

0000000000000050 <b_post_2_4>:
        ...

0000000000000058 <c_post_2_3>:
        ...

----

... where things are obviously nesting just fine.

Thanks,
Mark.