From mboxrd@z Thu Jan 1 00:00:00 1970 From: Boris Ostrovsky Subject: Re: linux-next: manual merge of the xen-tip tree with the tip tree Date: Wed, 12 Aug 2015 14:30:37 -0400 Message-ID: <55CB90CD.4000805@oracle.com> 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; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: Sender: linux-next-owner@vger.kernel.org To: Andy Lutomirski Cc: Peter Zijlstra , Stephen Rothwell , Jeremy Fitzhardinge , Konrad Rzeszutek Wilk , Stefano Stabellini , Xen Devel , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , "linux-next@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Andy Lutomirski , David Vrabel List-Id: xen-devel@lists.xenproject.org On 08/12/2015 02:26 PM, Andy Lutomirski wrote: > On Wed, Aug 12, 2015 at 11:17 AM, 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_ke= y >>>>> 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/n= mi.c:14: >>>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h: In fun= ction >>>>> =E2=80=98nmi_handle=E2=80=99: >>>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: war= ning: >>>>> asm >>>>> operand 0 probably doesn=E2=80=99t match constraints >>>>> /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: err= or: >>>>> 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 varian= t >>>> thereof tripped up more recent GCCs too. >>>> >>>> So its an __always_inline function, and both argument are always c= ompile >>>> 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, bu= t >>> its the next best I could come up with... >> >> No, it produces the same error. This is Fedora 13, btw, uses gcc 4.4= =2E4. > Is the problem just that it's being misdetected as supporting asm > goto? What does gcc -E say? static inline __attribute__((no_instrument_function))=20 __attribute__((always_inline)) bool arch_static_branch(struct static_ke= y=20 *key, bool branch) { do { asm goto("1:" ".byte " "0x0f,0x1f,0x44,0x00,0" "\n\t"=20 ".pushsection __jump_table, \"aw\" \n\t" " " ".balign 8" " " "\n\t" " = "=20 ".quad" " " "1b, %l[l_yes], %c0 \n\t" ".popsection \n\t" : : "i"=20 (&((char *)key)[branch]) : : l_yes); asm (""); } while (0) return false; l_yes: return true; } -boris