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([2001:8a0:f93a:3b00:e038:5cdc:b8bf:4653]) by smtp.gmail.com with ESMTPSA id e26-20020a5d595a000000b0022abcc1e3cesm1389410wri.116.2022.09.30.08.13.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 30 Sep 2022 08:13:03 -0700 (PDT) Subject: Re: [PATCH v3] gdb/arm: Handle lazy FPU register stacking To: =?UTF-8?Q?Torbj=c3=b6rn_SVENSSON?= , gdb-patches@sourceware.org Cc: tom@tromey.com, brobecker@adacore.com References: <20220927190944.201748-1-torbjorn.svensson@foss.st.com> From: Pedro Alves Message-ID: Date: Fri, 30 Sep 2022 16:13:02 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: <20220927190944.201748-1-torbjorn.svensson@foss.st.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-4.6 required=5.0 tests=BAYES_00, FREEMAIL_FORGED_FROMDOMAIN, FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS, KAM_DMARC_STATUS, NICE_REPLY_A, RCVD_IN_DNSWL_NONE, 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: Fri, 30 Sep 2022 15:13:07 -0000 On 2022-09-27 8:09 p.m., Torbjörn SVENSSON via Gdb-patches wrote: > Read LSPEN, ASPEN and LSPACT bits from FPCCR and use them for the > first exception frame. All frames after will always have the FPU > registers on the stack, regadless if lazy stacking is active or > inactive. See "Lazy context save of FP state", in B1.5.7, also ARM > AN298, supported by Cortex-M4F architecture for details on lazy FPU > register stacking. The same conditions are valid for other Cortex-M > cores with FPU. > > This patch has been verified on a STM32F4-Discovery board by: > a) writing a non-zero value (lets use 0x1122334455667788 as an > example) to all the D-registers in the main function > b) configured the SysTick to fire > c) in the SysTick_Handler, write some other value (lets use > 0x0022446688aaccee as an example) to one of the D-registers (D0 as > an example) and then do "SVC #0" > d) in the SVC_Handler, write some other value (lets use > 0x0099aabbccddeeff) to one of the D-registers (D0 as an example) > > In GDB, suspend the execution in the SVC_Handler function and compare > the value of the D-registers for the SVC_handler frame and the > SysTick_Handler frame. With the patch, the value of the modified > D-register (D0) should be the new value (0x009..eff) on the > SVC_Handler frame, and the intermediate value (0x002..cee) for the > SysTick_Handler frame. Now compare the D-register value for the > SysTick_Handler frame and the main frame. The main frame should > have the initial value (0x112..788). I suspect pasting a short GDB session here instead of prose may make this easier to grok. > static bool arm_debug; > +static bool force_fpu_regs_from_stack = false; > > /* Print an "arm" debug statement. */ > > @@ -3337,6 +3338,17 @@ struct frame_unwind arm_stub_unwind = { > arm_stub_unwind_sniffer > }; > > + > +/* The first time an exception frame is seen, the lazy stacking of the FPU > + registers should be considered. Any following exception frames should not > + consider the lazy stacking as the values will be put on the stack before > + branching to the nested exception handler. */ > +static void > +reset_force_fpu_regs_from_stack () > +{ > + force_fpu_regs_from_stack = false; > +} > + Is there really no self-contained way to tell that a frame is a "following frame after lazy stack has been enabled" ? E.g., can we look at the address in FPCAR and decide based on it, compared to the current frame address, or something like that?