Messages in this thread | | | Date | Mon, 9 Jul 2018 15:21:59 +0100 | From | Mark Rutland <> | Subject | Re: [PATCHv4 11/19] arm64: don't reload GPRs after apply_ssbd |
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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@arm.com> > > Acked-by: Marc Zyngier <marc.zyngier@arm.com> > > Acked-by: Catalin Marinas <catalin.marinas@arm.com> > > Cc: Will Deacon <will.deacon@arm.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.
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