From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: linux-next: manual merge of the xen-tip tree with the tip tree Date: Wed, 12 Aug 2015 11:26:41 -0700 Message-ID: References: <20150812150954.36bc605c@canb.auug.org.au> <55CB49CA.8050305@oracle.com> <20150812172105.GW16853@twins.programming.kicks-ass.net> <20150812174625.GH18673@twins.programming.kicks-ass.net> <55CB8DAD.6090508@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <55CB8DAD.6090508@oracle.com> Sender: linux-next-owner@vger.kernel.org To: Boris Ostrovsky , Peter Zijlstra Cc: Stephen Rothwell , Jeremy Fitzhardinge , Konrad Rzeszutek Wilk , Stefano Stabellini , Xen Devel , Thomas Gleixner , Ingo Molnar , linux-next@vger.kernel.org, linux-kernel@vger.kernel.org, Andy Lutomirski , David Vrabel List-Id: xen-devel@lists.xenproject.org One option might be to do the addition in assembly, i.e.: "i" (key), "i" (index) =2E.. and put the addition into the assembly source. On August 12, 2015 11:17:17 AM PDT, Boris Ostrovsky wrote: >On 08/12/2015 01:46 PM, Peter Zijlstra wrote: >> On Wed, Aug 12, 2015 at 07:21:05PM +0200, Peter Zijlstra wrote: >>> On Wed, Aug 12, 2015 at 09:27:38AM -0400, Boris Ostrovsky wrote: >>> >>>> Incidentally, 11276d53 ("locking/static_keys: Add a new static_key >>>> interface") breaks old-ish compilers (gcc version 4.4.4 20100503 >(Red Hat >>>> 4.4.4-2) (GCC)): >>>> >>>> >>>> >>>> CC arch/x86/kernel/nmi.o >>>> In file included from >>>> /home/build/linux-boris/include/linux/jump_label.h:109, >>>> from >>>> /home/build/linux-boris/arch/x86/include/asm/spinlock.h:5, >>>> from >/home/build/linux-boris/include/linux/spinlock.h:88, >>>> from >/home/build/linux-boris/arch/x86/kernel/nmi.c:14: >>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h: In >function >>>> =E2=80=98nmi_handle=E2=80=99: >>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: >warning: asm >>>> operand 0 probably doesn=E2=80=99t match constraints >>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: >error: >>>> impossible constraint in =E2=80=98asm=E2=80=99 >>>> make[3]: *** [arch/x86/kernel/nmi.o] Error 1 >>>> make[2]: *** [arch/x86/kernel] Error 2 >>>> make[1]: *** [arch/x86] Error 2 >>> Ugh bugger. >>> >>> I bet its that: &((char *)key)[branch] business, an earlier variant >>> thereof tripped up more recent GCCs too. >>> >>> So its an __always_inline function, and both argument are always >compile >>> time constants, @key is the address of an object in static storage >(a >>> global) and @branch is a simple 0/1 at the call site. >>> >>> Now we wish to compute (unsigned long)key + branch at compile/link >time >>> to feed to the assembler as an immediate, which should be possible, >>> given its all 'constants'. >>> >>> It just appears GCC is having a hard time with this. >>> >>> Let me see if I have a sufficiently old GCC around to play with. >> Could you feed the below to your compiler? Its a bit cumbersome, but >> its the next best I could come up with... > >No, it produces the same error. This is Fedora 13, btw, uses gcc 4.4.4= =2E > >-boris > > >> >> --- >> arch/x86/include/asm/jump_label.h | 56 >++++++++++++++++++++++++++++----------- >> 1 file changed, 41 insertions(+), 15 deletions(-) >> >> diff --git a/arch/x86/include/asm/jump_label.h >b/arch/x86/include/asm/jump_label.h >> index 28d7a857f9d1..76c769ae4200 100644 >> --- a/arch/x86/include/asm/jump_label.h >> +++ b/arch/x86/include/asm/jump_label.h >> @@ -16,15 +16,30 @@ >> # define STATIC_KEY_INIT_NOP GENERIC_NOP5_ATOMIC >> #endif >> =20 >> +struct foo { >> + u8 zero; >> + u8 one; >> +}; >> + >> static __always_inline bool arch_static_branch(struct static_key >*key, bool branch) >> { >> - asm_volatile_goto("1:" >> - ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t" >> - ".pushsection __jump_table, \"aw\" \n\t" >> - _ASM_ALIGN "\n\t" >> - _ASM_PTR "1b, %l[l_yes], %c0 \n\t" >> - ".popsection \n\t" >> - : : "i" (&((char *)key)[branch]) : : l_yes); >> + if (!branch) { >> + asm_volatile_goto("1:" >> + ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t" >> + ".pushsection __jump_table, \"aw\" \n\t" >> + _ASM_ALIGN "\n\t" >> + _ASM_PTR "1b, %l[l_yes], %c0 \n\t" >> + ".popsection \n\t" >> + : : "i" (&((struct foo *)key)->zero) : : l_yes); >> + } else { >> + asm_volatile_goto("1:" >> + ".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t" >> + ".pushsection __jump_table, \"aw\" \n\t" >> + _ASM_ALIGN "\n\t" >> + _ASM_PTR "1b, %l[l_yes], %c0 \n\t" >> + ".popsection \n\t" >> + : : "i" (&((struct foo *)key)->one) : : l_yes); >> + } >> =20 >> return false; >> l_yes: >> @@ -33,14 +48,25 @@ static __always_inline bool >arch_static_branch(struct static_key *key, bool bran >> =20 >> static __always_inline bool arch_static_branch_jump(struct >static_key *key, bool branch) >> { >> - asm_volatile_goto("1:" >> - ".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t" >> - "2:\n\t" >> - ".pushsection __jump_table, \"aw\" \n\t" >> - _ASM_ALIGN "\n\t" >> - _ASM_PTR "1b, %l[l_yes], %c0 \n\t" >> - ".popsection \n\t" >> - : : "i" (&((char *)key)[branch]) : : l_yes); >> + if (!branch) { >> + asm_volatile_goto("1:" >> + ".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t" >> + "2:\n\t" >> + ".pushsection __jump_table, \"aw\" \n\t" >> + _ASM_ALIGN "\n\t" >> + _ASM_PTR "1b, %l[l_yes], %c0 \n\t" >> + ".popsection \n\t" >> + : : "i" (&((struct foo *)key)->zero) : : l_yes); >> + } else { >> + asm_volatile_goto("1:" >> + ".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t" >> + "2:\n\t" >> + ".pushsection __jump_table, \"aw\" \n\t" >> + _ASM_ALIGN "\n\t" >> + _ASM_PTR "1b, %l[l_yes], %c0 \n\t" >> + ".popsection \n\t" >> + : : "i" (&((struct foo *)key)->one) : : l_yes); >> + } >> =20 >> return false; >> l_yes: --=20 Sent from my Android device with K-9 Mail. Please excuse my brevity.