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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de, linux-kernel@vger.kernel.org,
	joro@8bytes.org, boris.ostrovsky@oracle.com, jgross@suse.com,
	x86@kernel.org, mbenes@suse.com, rostedt@goodmis.org,
	dvyukov@google.com, elver@google.com
Subject: Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function
Date: Fri, 20 Aug 2021 12:31:07 -0700	[thread overview]
Message-ID: <20210820193107.omvshmsqbpxufzkc@treble> (raw)
In-Reply-To: <20210820192224.ytrr6ybuuwegbeov@treble>

On Fri, Aug 20, 2021 at 12:22:28PM -0700, Josh Poimboeuf wrote:
> On Thu, Jun 24, 2021 at 11:41:00AM +0200, Peter Zijlstra wrote:
> > The asm_cpu_bringup_and_idle() function is required to push the return
> > value on the stack in order to make ORC happy, but the only reason
> > objtool doesn't complain is because of a happy accident.
> > 
> > The thing is that asm_cpu_bringup_and_idle() doesn't return, so
> > validate_branch() never terminates and falls through to the next
> > function, which in the normal case is the hypercall_page. And that, as
> > it happens, is 4095 NOPs and a RET.
> > 
> > Make asm_cpu_bringup_and_idle() terminate on it's own, by making the
> > function it calls as a dead-end. This way we no longer rely on what
> > code happens to come after.
> > 
> > Fixes: c3881eb58d56 ("x86/xen: Make the secondary CPU idle tasks reliable")
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> 
> Looks right.  Only problem is, with my assembler I get this:
> 
>   arch/x86/kernel/head_64.o: warning: objtool: .text+0x5: unreachable instruction
> 
> Because gas insists on jumping over the page of nops...
> 
> 0000000000000000 <asm_cpu_bringup_and_idle>:
>        0:	e8 00 00 00 00       	callq  5 <asm_cpu_bringup_and_idle+0x5>
> 			1: R_X86_64_PLT32	cpu_bringup_and_idle-0x4
>        5:	e9 f6 0f 00 00       	jmpq   1000 <xen_hypercall_set_trap_table>
>        a:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>       11:	00 00 00 00 
>       15:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>       1c:	00 00 00 00 
>       20:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>       27:	00 00 00 00 
>       2b:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>       32:	00 00 00 00 
>       36:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
>       3d:	00 00 00 00 

Here's a fix:

From: Josh Poimboeuf <jpoimboe@redhat.com>
Subject: [PATCH] x86/xen: Move hypercall_page to top of the file

Because hypercall_page is page-aligned, the assembler inexplicably adds
an unreachable jump from after the end of the previous code to the
beginning of hypercall_page.

That confuses objtool, understandably.  It also creates significant text
fragmentation.  As a result, much of the object file is wasted text
(nops).

Move hypercall_page to the beginning of the file to both prevent the
text fragmentation and avoid the dead jump instruction.

$ size /tmp/head_64.before.o /tmp/head_64.after.o
   text	   data	    bss	    dec	    hex	filename
  10924	 307252	   4096	 322272	  4eae0	/tmp/head_64.before.o
   6823	 307252	   4096	 318171	  4dadb	/tmp/head_64.after.o

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/xen/xen-head.S | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index cb6538ae2fe0..488944d6d430 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -20,6 +20,23 @@
 #include <xen/interface/xen-mca.h>
 #include <asm/xen/interface.h>
 
+.pushsection .text
+	.balign PAGE_SIZE
+SYM_CODE_START(hypercall_page)
+	.rept (PAGE_SIZE / 32)
+		UNWIND_HINT_FUNC
+		.skip 31, 0x90
+		ret
+	.endr
+
+#define HYPERCALL(n) \
+	.equ xen_hypercall_##n, hypercall_page + __HYPERVISOR_##n * 32; \
+	.type xen_hypercall_##n, @function; .size xen_hypercall_##n, 32
+#include <asm/xen-hypercalls.h>
+#undef HYPERCALL
+SYM_CODE_END(hypercall_page)
+.popsection
+
 #ifdef CONFIG_XEN_PV
 	__INIT
 SYM_CODE_START(startup_xen)
@@ -64,23 +81,6 @@ SYM_CODE_END(asm_cpu_bringup_and_idle)
 #endif
 #endif
 
-.pushsection .text
-	.balign PAGE_SIZE
-SYM_CODE_START(hypercall_page)
-	.rept (PAGE_SIZE / 32)
-		UNWIND_HINT_FUNC
-		.skip 31, 0x90
-		ret
-	.endr
-
-#define HYPERCALL(n) \
-	.equ xen_hypercall_##n, hypercall_page + __HYPERVISOR_##n * 32; \
-	.type xen_hypercall_##n, @function; .size xen_hypercall_##n, 32
-#include <asm/xen-hypercalls.h>
-#undef HYPERCALL
-SYM_CODE_END(hypercall_page)
-.popsection
-
 	ELFNOTE(Xen, XEN_ELFNOTE_GUEST_OS,       .asciz "linux")
 	ELFNOTE(Xen, XEN_ELFNOTE_GUEST_VERSION,  .asciz "2.6")
 	ELFNOTE(Xen, XEN_ELFNOTE_XEN_VERSION,    .asciz "xen-3.0")
-- 
2.31.1


  reply	other threads:[~2021-08-20 19:31 UTC|newest]

Thread overview: 101+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-24  9:40 [PATCH v2 00/24] objtool/x86: noinstr vs PARAVIRT Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function Peter Zijlstra
2021-06-24 10:49   ` Juergen Gross
2021-06-30 11:47   ` Miroslav Benes
2021-08-20 19:22   ` Josh Poimboeuf
2021-08-20 19:31     ` Josh Poimboeuf [this message]
2021-08-23  8:40       ` Juergen Gross
2021-08-30  5:55         ` Juergen Gross
2021-08-30  7:48           ` Peter Zijlstra
2021-08-30  7:56             ` Juergen Gross
2021-09-15 15:49       ` [tip: objtool/core] x86/xen: Move hypercall_page to top of the file tip-bot2 for Josh Poimboeuf
2021-09-15 15:49   ` [tip: objtool/core] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 02/24] objtool: Introduce CFI hash Peter Zijlstra
2021-06-30 12:45   ` Miroslav Benes
2021-07-01  8:35     ` Peter Zijlstra
2021-07-01  8:53       ` Miroslav Benes
2021-08-20 22:27   ` Josh Poimboeuf
2021-08-25 10:13     ` Peter Zijlstra
2021-08-25 12:49       ` Peter Zijlstra
2021-08-25 10:15     ` Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 03/24] objtool: Handle __sanitize_cov*() tail calls Peter Zijlstra
2021-06-25 11:05   ` Marco Elver
2021-06-28  9:07     ` Peter Zijlstra
2021-06-25 13:38   ` Steven Rostedt
2021-06-26  8:18     ` Peter Zijlstra
2021-06-29  1:07       ` Steven Rostedt
2021-08-20 23:17   ` Josh Poimboeuf
2021-08-25 10:19     ` Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 04/24] x86/kvm: Always inline sev_*guest() Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 05/24] x86/kvm: Always inline vmload() / vmsave() Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 06/24] x86: Always inline context_tracking_guest_enter() Peter Zijlstra
2021-08-20 23:22   ` Josh Poimboeuf
2021-08-20 23:34     ` Josh Poimboeuf
2021-08-25 12:16       ` Peter Zijlstra
     [not found]         ` <YSZxtBHNJzoUbqdE@ravnborg.org>
2021-08-26  4:10           ` Josh Poimboeuf
2021-08-25 13:38       ` Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 07/24] x86/kvm: Always inline to_svm() Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 08/24] x86/kvm: Always inline evmcs_write64() Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 09/24] x86: Always inline ip_within_syscall_gap() Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 10/24] x86/sev: Fix noinstr for vc_ghcb_invalidate() Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 11/24] locking/lockdep: Avoid RCU-induced noinstr fail Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 12/24] x86/paravirt: Mark arch_local_irq_*() __always_inline Peter Zijlstra
2021-06-24 10:51   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 13/24] x86/paravirt: Use PVOP_* for paravirt calls Peter Zijlstra
2021-06-24 10:52   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 14/24] x86/xen: Make read_cr2() noinstr Peter Zijlstra
2021-06-24 10:55   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 15/24] x86/xen: Make write_cr2() noinstr Peter Zijlstra
2021-06-24 10:56   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 16/24] x86/xen: Make get_debugreg() noinstr Peter Zijlstra
2021-06-24 10:57   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 17/24] x86/xen: Make set_debugreg() noinstr Peter Zijlstra
2021-06-24 10:59   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 18/24] x86/xen: Make save_fl() noinstr Peter Zijlstra
2021-06-24 11:07   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 19/24] x86/xen: Make hypercall_page noinstr Peter Zijlstra
2021-06-24 11:08   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 20/24] x86/xen: Make irq_enable() noinstr Peter Zijlstra
2021-06-24 11:09   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 21/24] x86/xen: Make irq_disable() noinstr Peter Zijlstra
2021-06-24 11:10   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 22/24] x86/xen: Mark xen_force_evtchn_callback() noinstr Peter Zijlstra
2021-06-24 11:11   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 23/24] x86/xen: Rework the xen_{cpu,irq,mmu}_ops[] arrays Peter Zijlstra
2021-06-24 12:12   ` Juergen Gross
2021-09-15 15:49   ` [tip: objtool/core] x86/xen: Rework the xen_{cpu,irq,mmu}_opsarrays tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24  9:41 ` [PATCH v2 24/24] objtool: Support pv_opsindirect calls for noinstr Peter Zijlstra
2021-09-15 15:49   ` [tip: objtool/core] " tip-bot2 for Peter Zijlstra
2021-09-17 12:58   ` tip-bot2 for Peter Zijlstra
2021-06-24 13:05 ` [PATCH v2 00/24] objtool/x86: noinstr vs PARAVIRT Steven Rostedt

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