From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.freebsd.org (mx2.freebsd.org [96.47.72.81]) by sourceware.org (Postfix) with ESMTPS id 5B39A38582B8 for ; Wed, 5 Oct 2022 16:49:07 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 5B39A38582B8 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=FreeBSD.org Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=FreeBSD.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) client-signature RSA-PSS (4096 bits)) (Client CN "mx1.freebsd.org", Issuer "R3" (verified OK)) by mx2.freebsd.org (Postfix) with ESMTPS id 4MjLBP1QHjz3xSf; Wed, 5 Oct 2022 16:49:01 +0000 (UTC) (envelope-from jhb@FreeBSD.org) Received: from smtp.freebsd.org (smtp.freebsd.org [IPv6:2610:1c1:1:606c::24b:4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256 client-signature RSA-PSS (4096 bits) client-digest SHA256) (Client CN "smtp.freebsd.org", Issuer "R3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4MjLBP0BSSz3s2J; 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Wed, 5 Oct 2022 16:49:00 +0000 (UTC) (envelope-from jhb@FreeBSD.org) Message-ID: Date: Wed, 5 Oct 2022 09:48:58 -0700 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.13.1 Subject: Re: [PATCH] [Arm] Remove dead FPA code Content-Language: en-US To: Luis Machado , Pedro Alves , gdb-patches@sourceware.org, David Spickett References: <20220920123012.189293-1-luis.machado@arm.com> <73479562-ab47-dfbf-aadc-7a2203c0f0e4@FreeBSD.org> <56653c70-593a-4b8d-ddf7-52f7dd0608f7@arm.com> <1946bc74-8270-23c4-9483-702b9dbc03de@FreeBSD.org> <3cb2d818-83e8-4e2d-5e1b-9b555d1a2217@arm.com> From: John Baldwin In-Reply-To: <3cb2d818-83e8-4e2d-5e1b-9b555d1a2217@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=freebsd.org; s=dkim; t=1664988541; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=DSNU+YKMyBx1gW9GSjHd6duBCPEZgvhVj1pLX5ejqZs=; b=W6kSzkvNeTr6gm+7Fw4W6wYT7cgutoFtaAbeFAB5XFKfLy7SItV1X08ji6csEWZRN/u36N ws+ZW4ty0g/T4MyMSFrk/FiBIXWgQliyKCYksbTjCNJ02o2sjDhr8jO28wMqnDy71Qhr68 fB/pkS0fg/eQse00wpU6zfjoYjY0ZX3/CiH2gqVEJpNtH6zrHmhjRv+NIZajedWnhVJboW ikVGtnLNEwduVAP26qCDrjz8EKTFfKUfdudnb4YIMRZ0ulab4Ghxqs7yzGnX3ZzOGjZtyW Oj3MLVBhPdxCH3NIASJeqt0uiVFLDQZkvEoHFPjcR67gEkCOYwEBFinuvWPDLg== ARC-Seal: i=1; s=dkim; d=freebsd.org; t=1664988541; a=rsa-sha256; cv=none; b=N9GIZLGXeRMsZ0kPYWwBuotKnnchPAxwY5d8O01Yzod5n2J3tL6wGc4PZS7BLW2i1P9c/f h1zYPtDwvQWrqTZ4qYjVy9fqcY33C4cOvsIgoqOadCQVRu9GwRYsh/9LHG0Jo8dwHdFPL9 EOXIzkNGNYZ9v9W8hgZsyTErnwfGuvPHnq2HeJSr+X5FTEo3VrzeSUko6LZMBVga+hePxa J8cE2JGTuUqgxppCKaQRxUcbFYsnSl1onbaKXSAGT+dbmlDMOZOtPCSlZQTYAlO4jWaXW8 NixKFg3VczoNf1x5hCIU1Ix4C0OiJ0zSyrLu3jqocCpNr1hxgeycC3VpAb7GTQ== ARC-Authentication-Results: i=1; mx1.freebsd.org; none X-Spam-Status: No, score=-10.0 required=5.0 tests=BAYES_00, BODY_8BITS, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, NICE_REPLY_A, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 05 Oct 2022 16:49:09 -0000 On 10/5/22 1:26 AM, Luis Machado wrote: > On 10/4/22 22:36, John Baldwin wrote: >> On 10/4/22 10:43 AM, Luis Machado wrote: >>> On 10/4/22 18:08, John Baldwin wrote: >>>> On 10/4/22 1:43 AM, Luis Machado via Gdb-patches wrote: >>>>> On 10/3/22 20:16, Pedro Alves wrote: >>>>>> On 2022-09-20 1:30 p.m., Luis Machado via Gdb-patches wrote: >>>>>> >>>>>>> diff --git a/gdb/arch/arm.h b/gdb/arch/arm.h >>>>>>> index 36757493406..74a6ba93bc7 100644 >>>>>>> --- a/gdb/arch/arm.h >>>>>>> +++ b/gdb/arch/arm.h >>>>>>> @@ -44,11 +44,6 @@ enum gdb_regnum { >>>>>>>       ARM_SP_REGNUM = 13,        /* Contains address of top of stack */ >>>>>>>       ARM_LR_REGNUM = 14,        /* address to return to from a function call */ >>>>>>>       ARM_PC_REGNUM = 15,        /* Contains program counter */ >>>>>>> -  /* F0..F7 are the fp registers for the (obsolete) FPA architecture.  */ >>>>>> >>>>>> Shouldn't we leave behind a comment explaining why there's a hole between 15 and 25? >>>>> >>>>> I pondered about this a bit more, and I think we should close the gap and bring CPSR down to >>>>> 16, its "natural" position. It is what linux uses for user_regs as well, in gdb/arch/arm-linux.h: >>>>> >>>>> /* The index to access CSPR in user_regs defined in GLIBC.  */ >>>>> #define ARM_CPSR_GREGNUM 16 >>>>> >>>>>> >>>>>> IIRC the numbers can't be changed since we need to handle the case when the target >>>>>> doesn't send an xml tdesc, so it'd be good to help future readers understand why >>>>>> there's a hole. >>>>> >>>>> That's correct. Though a 32-bit Arm target that doesn't support XML descriptions these days is not very >>>>> common. I haven't seen one in a while. >>>>> >>>>> I'm willing to declare old 32-bit Arm targets that don't send XML target descriptions back as unsupported. >>>>> >>>>> To that effect, I suppose we should add a note to make it more explicit. >>>>> >>>>> More below. >>>> >>>> FWIW, the GDB stub in FreeBSD's kernel does not use XML target descriptions >>>> for any architectures, but it also only tends to do GPRs and not any floating >>>> point.  For 32-bit ARM it does not report any register values higher than >>>> number 15 (PC), so it would not be affected by changing this. >>> >>> Does it care about CPSR and/or XPSR? Could you please give it a try to see if the defaults would suit it just fine? >> >> Hmm, I misread and it does care about CPSR for the current thread.  The >> relevant code is here (From https://cgit.freebsd.org/src/tree/sys/arm/arm/gdb_machdep.c): >> >> void * >> gdb_cpu_getreg(int regnum, size_t *regsz) >> { >> >>     *regsz = gdb_cpu_regsz(regnum); >> >>     if (kdb_thread == curthread) { >>         if (regnum < 13) >>             return (&kdb_frame->tf_r0 + regnum); >>         if (regnum == 13) >>             return (&kdb_frame->tf_svc_sp); >>         if (regnum == 14) >>             return (&kdb_frame->tf_svc_lr); >>         if (regnum == 15) >>             return (&kdb_frame->tf_pc); >>         if (regnum == 25) >>             return (&kdb_frame->tf_spsr); >>     } >> >>     switch (regnum) { >>     case 4:  return (&kdb_thrctx->pcb_regs.sf_r4); >>     case 5:  return (&kdb_thrctx->pcb_regs.sf_r5); >>     case 6:  return (&kdb_thrctx->pcb_regs.sf_r6); >>     case 7:  return (&kdb_thrctx->pcb_regs.sf_r7); >>     case 8:  return (&kdb_thrctx->pcb_regs.sf_r8); >>     case 9:  return (&kdb_thrctx->pcb_regs.sf_r9); >>     case 10:  return (&kdb_thrctx->pcb_regs.sf_r10); >>     case 11:  return (&kdb_thrctx->pcb_regs.sf_r11); >>     case 12:  return (&kdb_thrctx->pcb_regs.sf_r12); >>     case 13:  stacktest = kdb_thrctx->pcb_regs.sf_sp + 5 * 4; >>           return (&stacktest); >>     case 15: >>           /* >>            * On context switch, the PC is not put in the PCB, but >>            * we can retrieve it from the stack. >>            */ >>           if (kdb_thrctx->pcb_regs.sf_sp > KERNBASE) { >>               kdb_thrctx->pcb_regs.sf_pc = *(register_t *) >>                   (kdb_thrctx->pcb_regs.sf_sp + 4 * 4); >>               return (&kdb_thrctx->pcb_regs.sf_pc); >>           } >>     } >> >>     return (NULL); >> } >> >> The 'kdb_thread == curthread' case is when a thread enters the debugger due >> to a crash or breakpoint, etc.  We do return CPSR for that thread, but we do >> not return it for other threads.  It looks like we do also know the FPA register >> size so that we return enough "xx" bytes in the 'g' reply to mark the FP >> registers as unavailable so that we can return the value of CPSR in the 'g' >> reply. >> >> From https://cgit.freebsd.org/src/tree/sys/arm/include/gdb_machdep.h: >> >> #define    GDB_NREGS    26 >> #define    GDB_REG_SP    13 >> #define    GDB_REG_LR    14 >> #define    GDB_REG_PC    15 >> >> static __inline size_t >> gdb_cpu_regsz(int regnum) >> { >>     /* >>      * GDB expects the FPA registers f0-f7, each 96 bits wide, to be placed >>      * in between the PC and CSPR in response to a "g" packet. >>      */ >>     return (regnum >= 16 && regnum <= 23 ? 12 : sizeof(int)); >> } >> >> NetBSD's kernel seems to have similar knowledge: >> >> http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/arch/arm/include/db_machdep.h?rev=1.28&content-type=text/x-cvsweb-markup&only_with_tag=MAIN >> >> (The kgdb bits near the bottom) >> >> Linux's kernel also seems to maybe hardcode this knowledge as well: >> >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/kernel/kgdb.c#n21 >> > > Yeah, that's what I was worried about. Register discoveries without XML are not great, and more recently debugging stubs have been > exposing more system registers. Having to consider FPA (which was *removed* 10 years ago from GCC, but fell in disuse before then) is not > acceptable at this point. > > If those debugging stubs want to skip XML, I think it would be reasonable for them to at least update the expected 'g' packet to contain just > the basic registers, with CPSR as 16. > > That might need some coordination. I can coordinate this from the Linux Kernel's side, but I never dealt with the BSD kernels. I can certainly deal with updating FreeBSD. I can try to find someone over in NetBSD land. Honestly, the simplest approach might be to start exporting XML from the kernel. However, an even simpler workaround for now might be having a knob that can be set for older debugger binaries that still places CPSR at the old register number vs the newer one. -- John Baldwin