From: Juergen Gross <jgross@suse.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>,
Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de, linux-kernel@vger.kernel.org,
joro@8bytes.org, boris.ostrovsky@oracle.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: Mon, 23 Aug 2021 10:40:51 +0200 [thread overview]
Message-ID: <fc90e972-f81f-3706-b13a-988b16264650@suse.com> (raw)
In-Reply-To: <20210820193107.omvshmsqbpxufzkc@treble>
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On 20.08.21 21:31, Josh Poimboeuf wrote:
> 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
Reviewed-by: Juergen Gross <jgross@suse.com>
Juergen
> ---
> 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")
>
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next prev parent reply other threads:[~2021-08-23 8:40 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
2021-08-23 8:40 ` Juergen Gross [this message]
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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