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Wed, 12 Jun 2024 19:48:43 -0700 (PDT) From: To: "'Matheus Tavares Bernardino'" Cc: , , , , , , , , , References: <20240612164239.90276-1-ltaylorsimpson@gmail.com> <20240612183008.4123155-1-quic_mathbern@quicinc.com> In-Reply-To: <20240612183008.4123155-1-quic_mathbern@quicinc.com> Subject: RE: [PATCH] Hexagon: lldb read/write predicate registers p0/p1/p2/p3 Date: Wed, 12 Jun 2024 20:48:42 -0600 Message-ID: <066c01dabd3c$343be980$9cb3bc80$@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit X-Mailer: Microsoft Outlook 16.0 Thread-Index: AQHMHiXUmCnzvbCFbV64tIsn2swJnwIEBIVEsdIVtQA= Content-Language: en-us Received-SPF: pass client-ip=2607:f8b0:4864:20::d36; envelope-from=ltaylorsimpson@gmail.com; helo=mail-io1-xd36.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org > -----Original Message----- > From: Matheus Tavares Bernardino > Sent: Wednesday, June 12, 2024 12:30 PM > To: ltaylorsimpson@gmail.com > Cc: qemu-devel@nongnu.org; bcain@quicinc.com; tedwood@quicinc.com; > alex.bennee@linaro.org; quic_mathbern@quicinc.com; > sidneym@quicinc.com; quic_mliebel@quicinc.com; > richard.henderson@linaro.org; philmd@linaro.org; ale@rev.ng; anjo@rev.ng > Subject: Re: [PATCH] Hexagon: lldb read/write predicate registers > p0/p1/p2/p3 > > On Wed, 12 Jun 2024 10:42:39 -0600 Taylor Simpson > wrote: > > > > diff --git a/target/hexagon/gdbstub.c b/target/hexagon/gdbstub.c index > > 502c6987f0..e67e627fc9 100644 > > --- a/target/hexagon/gdbstub.c > > +++ b/target/hexagon/gdbstub.c > > @@ -56,6 +64,15 @@ int hexagon_gdb_write_register(CPUState *cs, > uint8_t *mem_buf, int n) > > return sizeof(target_ulong); > > } > > > > + n -= TOTAL_PER_THREAD_REGS; > > + > > + if (n < NUM_PREGS) { > > + env->pred[n] = ldtul_p(mem_buf); > > + return sizeof(uint8_t); > > I wonder, shouldn't this be sizeof(target_ulong) since we wrote a > target_ulong? Good question. >From the architecture point of view, predicates are 8 bits (Section 2.2.5 of the v73 Hexagon PRM). However, we model them in QEMU as target_ulong because TCG variables must be either 32 bits or 64 bits. There isn't an option for 8 bits. Whenever we write to a predicate, do "and" with 0xff first to ensure there are only 8 bits written (see gen_log_pred_write in target/hexagon/genptr.c). I did some more digging and here is what I found: - Since we have bitsize="8" in hexagon-core.xml, lldb will reject any attempt to write something larger. (lldb) reg write p1 0x1ff error: Failed to write register 'p1' with value '0x1ff': value 0x1ff is too large to fit in a 1 byte unsigned integer value - For the lldb "reg write" command, the return value from hexagon_gdb_write_register isn't used. - The only place the return value is used is in handle_write_all_regs. This function is called in response to a "G" packet from the debugger. I don't know if/when lldb uses this packet, but it seems like it would count on it being 8 bits since that's what is in hexagon-core.xml. Ted , when would lldb generate a "G" packet, and what assumptions will it make about the size of predicate registers? Thanks, Taylor