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Mon, 23 Aug 2021 08:40:52 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap1.suse-dmz.suse.de with ESMTPSA id 9QzaEhRfI2EWZgAAGKfGzw (envelope-from ); Mon, 23 Aug 2021 08:40:52 +0000 Subject: Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function To: Josh Poimboeuf , Peter Zijlstra 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 References: <20210624094059.886075998@infradead.org> <20210624095147.693801717@infradead.org> <20210820192224.ytrr6ybuuwegbeov@treble> <20210820193107.omvshmsqbpxufzkc@treble> From: Juergen Gross Message-ID: Date: Mon, 23 Aug 2021 10:40:51 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210820193107.omvshmsqbpxufzkc@treble> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="qsNSH97XrotVcJ8VdwzuG0l96GXoTnoj4" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --qsNSH97XrotVcJ8VdwzuG0l96GXoTnoj4 Content-Type: multipart/mixed; boundary="v7O9DOxhYbMgJdmmM1ggRcf6MAvWYS29s"; protected-headers="v1" From: Juergen Gross To: Josh Poimboeuf , Peter Zijlstra 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 Message-ID: Subject: Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function References: <20210624094059.886075998@infradead.org> <20210624095147.693801717@infradead.org> <20210820192224.ytrr6ybuuwegbeov@treble> <20210820193107.omvshmsqbpxufzkc@treble> In-Reply-To: <20210820193107.omvshmsqbpxufzkc@treble> --v7O9DOxhYbMgJdmmM1ggRcf6MAvWYS29s Content-Type: multipart/mixed; boundary="------------13BCEB8768998A12ACC48350" Content-Language: en-US This is a multi-part message in MIME format. --------------13BCEB8768998A12ACC48350 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable 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 retur= n >>> 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, a= s >>> 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 reli= able") >>> Signed-off-by: Peter Zijlstra (Intel) >> >> 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 : >> 0: e8 00 00 00 00 callq 5 >> 1: R_X86_64_PLT32 cpu_bringup_and_idle-0x4 >> 5: e9 f6 0f 00 00 jmpq 1000 >> 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 >=20 > Here's a fix: >=20 > From: Josh Poimboeuf > Subject: [PATCH] x86/xen: Move hypercall_page to top of the file >=20 > 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. >=20 > That confuses objtool, understandably. It also creates significant tex= t > fragmentation. As a result, much of the object file is wasted text > (nops). >=20 > Move hypercall_page to the beginning of the file to both prevent the > text fragmentation and avoid the dead jump instruction. >=20 > $ 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 >=20 > Signed-off-by: Josh Poimboeuf Juergen > --- > arch/x86/xen/xen-head.S | 34 +++++++++++++++++----------------- > 1 file changed, 17 insertions(+), 17 deletions(-) >=20 > 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 > #include > =20 > +.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 > +#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 > =20 > -.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 > -#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") >=20 --------------13BCEB8768998A12ACC48350 Content-Type: application/pgp-keys; name="OpenPGP_0xB0DE9DD628BF132F.asc" Content-Transfer-Encoding: quoted-printable Content-Description: OpenPGP public key Content-Disposition: attachment; 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