From patchwork Thu Sep 20 20:31:46 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: John Baldwin X-Patchwork-Id: 29494 Received: (qmail 59633 invoked by alias); 20 Sep 2018 20:31:53 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Delivered-To: mailing list gdb-patches@sourceware.org Received: (qmail 59616 invoked by uid 89); 20 Sep 2018 20:31:53 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-25.2 required=5.0 tests=AWL, BAYES_00, GIT_PATCH_0, GIT_PATCH_1, GIT_PATCH_2, GIT_PATCH_3, SPF_HELO_PASS, SPF_SOFTFAIL autolearn=ham version=3.3.2 spammy=sk:_initia, H*f:sk:CAFyWVa, core_addr, H*UA:Intel X-HELO: mail.baldwin.cx Received: from bigwig.baldwin.cx (HELO mail.baldwin.cx) (96.47.65.170) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Thu, 20 Sep 2018 20:31:49 +0000 Received: from John-Baldwins-MacBook-Pro-2.local (ralph.baldwin.cx [66.234.199.215]) by mail.baldwin.cx (Postfix) with ESMTPSA id 4FC9510A87D; Thu, 20 Sep 2018 16:31:47 -0400 (EDT) Subject: Re: [PATCH 2/4] Fall back to a default value of 0 for the MISA register. From: John Baldwin To: Jim Wilson References: <20180919231950.22634-1-jhb@FreeBSD.org> <20180919231950.22634-3-jhb@FreeBSD.org> <0081bdf8-04cb-f6b7-d80a-d9a878d0a3ab@FreeBSD.org> Cc: gdb-patches@sourceware.org, Andrew Burgess , Palmer Dabbelt Message-ID: Date: Thu, 20 Sep 2018 13:31:46 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <0081bdf8-04cb-f6b7-d80a-d9a878d0a3ab@FreeBSD.org> X-IsSubscribed: yes On 9/19/18 5:40 PM, John Baldwin wrote: > Whether or not 'C' is present is a bit more subtle as it isn't something > you could infer just from a register being present or not. On the other > hand, for breakpoints, I wonder if a more straightforward approach would > be to examine the opcode of the existing instruction and use that to > decide which breakpoint to use? That is, read the first byte of the > existing instruction and if the low 2 bits indicate a 16-bit instruction > use C.BREAK and otherwise use BREAK? This seems to work for me in my testing (I used 'stepi' to step across a set of instructions with a mix of regular and 'C' instructions and it worked correctly). You can use 'set debug riscv breakpoints 1' to see what it decides each time it sets a breakpoint. I haven't yet tested if this means I can remove the MISA patch, but I suspect it does as the only place that needs MISA after this is the riscv_isa_flen() function. Author: John Baldwin Date: Thu Sep 20 10:12:22 2018 -0700 Use the existing instruction to determine the RISC-V breakpoint kind. RISC-V supports instructions of varying lengths. Standard existing instructions in the base ISA are 4 bytes in length, but the 'C' extension adds support for compressed, 2 byte instructions. RISC-V supports two different breakpoint instructions: EBREAK is a 4 byte instruction in the base ISA, and C.EBREAK is a 2 byte instruction only available on processors implementing the 'C' extension. Using EBREAK to set breakpoints on compressed instructions causes problems as the second half of EBREAK will overwrite the first 2 bytes of the following instruction breaking other threads in the process if their PC is the following instruction. Thus, breakpoints on compressed instructions need to use C.EBREAK instead of EBREAK. Previously, the riscv architecture checked the MISA register to determine if the 'C' extension was available. If so, it used C.EBREAK for all breakpoints. However, the MISA register is not necessarily available to supervisor mode operating systems. While native targets could provide a fake MISA register value, this patch instead examines the existing instruction at a breakpoint target to determine which breakpoint instruction to use. If the existing instruction is a compressed instruction, C.EBREAK is used, otherwise EBREAK is used. gdb/ChangeLog: * riscv-tdep.c (show_use_compressed_breakpoints): Remove 'additional_info' and related logic. (riscv_debug_breakpoints): New variable. (riscv_breakpoint_kind_from_pc): Use the length of the existing instruction to determine the breakpoint kind. (_initialize_riscv_tdep): Add 'set/show debug riscv breakpoints' flag. Update description of 'set/show riscv use-compressed-breakpoints' flag. diff --git a/gdb/ChangeLog b/gdb/ChangeLog index 75032b7a4d..df4561d319 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,14 @@ +2018-09-20 John Baldwin + + * riscv-tdep.c (show_use_compressed_breakpoints): Remove + 'additional_info' and related logic. + (riscv_debug_breakpoints): New variable. + (riscv_breakpoint_kind_from_pc): Use the length of the existing + instruction to determine the breakpoint kind. + (_initialize_riscv_tdep): Add 'set/show debug riscv breakpoints' + flag. Update description of 'set/show riscv + use-compressed-breakpoints' flag. + 2018-09-19 John Baldwin * Makefile.in (ALLDEPFILES): Add riscv-fbsd-nat.c. diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c index 4b385ed5da..cd8dbaf9f6 100644 --- a/gdb/riscv-tdep.c +++ b/gdb/riscv-tdep.c @@ -210,20 +210,9 @@ show_use_compressed_breakpoints (struct ui_file *file, int from_tty, struct cmd_list_element *c, const char *value) { - const char *additional_info; - struct gdbarch *gdbarch = target_gdbarch (); - - if (use_compressed_breakpoints == AUTO_BOOLEAN_AUTO) - if (riscv_has_feature (gdbarch, 'C')) - additional_info = _(" (currently on)"); - else - additional_info = _(" (currently off)"); - else - additional_info = ""; - fprintf_filtered (file, _("Debugger's use of compressed breakpoints is set " - "to %s%s.\n"), value, additional_info); + "to %s.\n"), value); } /* The set and show lists for 'set riscv' and 'show riscv' prefixes. */ @@ -284,6 +273,11 @@ show_riscv_debug_variable (struct ui_file *file, int from_tty, c->name, value); } +/* When this is set to non-zero debugging information about breakpoint + kinds will be printed. */ + +static unsigned int riscv_debug_breakpoints = 0; + /* When this is set to non-zero debugging information about inferior calls will be printed. */ @@ -426,7 +420,22 @@ riscv_breakpoint_kind_from_pc (struct gdbarch *gdbarch, CORE_ADDR *pcptr) { if (use_compressed_breakpoints == AUTO_BOOLEAN_AUTO) { - if (riscv_has_feature (gdbarch, 'C')) + enum bfd_endian byte_order = gdbarch_byte_order_for_code (gdbarch); + gdb_byte buf[1]; + int status; + + /* Read the opcode byte to determine the instruction length. */ + status = target_read_memory (*pcptr, buf, 1); + if (status) + memory_error (TARGET_XFER_E_IO, *pcptr); + + if (riscv_debug_breakpoints) + fprintf_unfiltered + (gdb_stdlog, + "Using %s for breakpoint at %s (instruction length %d)\n", + riscv_insn_length (buf[0]) == 2 ? "C.EBREAK" : "EBREAK", + paddress (gdbarch, *pcptr), riscv_insn_length (buf[0])); + if (riscv_insn_length (buf[0]) == 2) return 2; else return 4; @@ -2945,6 +2954,16 @@ _initialize_riscv_tdep (void) &showdebugriscvcmdlist, "show debug riscv ", 0, &showdebuglist); + add_setshow_zuinteger_cmd ("breakpoints", class_maintenance, + &riscv_debug_breakpoints, _("\ +Set riscv breakpoint debugging."), _("\ +Show riscv breakpoint debugging."), _("\ +When non-zero, print debugging information for the riscv specific parts\n\ +of the breakpoint mechanism."), + NULL, + show_riscv_debug_variable, + &setdebugriscvcmdlist, &showdebugriscvcmdlist); + add_setshow_zuinteger_cmd ("infcall", class_maintenance, &riscv_debug_infcall, _("\ Set riscv inferior call debugging."), _("\ @@ -2982,9 +3001,9 @@ of the stack unwinding mechanism."), Set debugger's use of compressed breakpoints."), _(" \ Show debugger's use of compressed breakpoints."), _("\ Debugging compressed code requires compressed breakpoints to be used. If\n \ -left to 'auto' then gdb will use them if $misa indicates the C extension\n \ -is supported. If that doesn't give the correct behavior, then this option\n\ -can be used."), +left to 'auto' then gdb will use them if the existing instruction is a\n \ +compressed instruction. If that doesn't give the correct behavior, then\n \ +this option can be used."), NULL, show_use_compressed_breakpoints, &setriscvcmdlist,