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Thu, 31 Jul 2025 14:21:56 -0700 (PDT) Received: from [192.168.1.87] ([38.41.223.211]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-76bcce6f6d8sm2468307b3a.2.2025.07.31.14.21.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 31 Jul 2025 14:21:55 -0700 (PDT) Message-ID: <006a95c0-7a44-43db-96f8-271e5dcd7523@linaro.org> Date: Thu, 31 Jul 2025 14:21:55 -0700 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH for-10.1] target/arm: Reinstate bogus AArch32 DBGDTRTX register for migration compat Content-Language: en-US To: Peter Maydell , qemu-arm@nongnu.org, qemu-devel@nongnu.org Cc: Fabiano Rosas References: <20250731134338.250203-1-peter.maydell@linaro.org> From: Pierrick Bouvier In-Reply-To: <20250731134338.250203-1-peter.maydell@linaro.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::42b; envelope-from=pierrick.bouvier@linaro.org; helo=mail-pf1-x42b.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, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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 On 7/31/25 6:43 AM, Peter Maydell wrote: > In commit 655659a74a we fixed some bugs in the encoding of the > Debug Communications Channel registers, including that we were > incorrectly exposing an AArch32 register at p14, 3, c0, c5, 0. > > Unfortunately removing a register is a break of forwards migration > compatibility for TCG, because we will fail the migration if the > source QEMU passes us a cpreg which the destination QEMU does not > have. We don't have a mechanism for saying "it's OK to ignore this > sysreg in the inbound data", so for the 10.1 release reinstate the > incorrect AArch32 register. > > (We probably have had other cases in the past of breaking migration > compatibility like this, but we didn't notice because we didn't test > and in any case not that many people care about TCG migration > compatibility. KVM migration compat is not affected because for KVM > we treat the kernel as the source of truth for what system registers > are present.) > > Fixes: 655659a74a36b ("target/arm: Correct encoding of Debug Communications Channel registers") > Reported-by: Fabiano Rosas > Signed-off-by: Peter Maydell > --- > Fabiano's suggestion for a migration compat mechanism is > https://patchew.org/QEMU/20250730205245.2118-1-farosas@suse.de/20250730205245.2118-2-farosas@suse.de/ > --- > target/arm/debug_helper.c | 29 +++++++++++++++++++++++++++++ > 1 file changed, 29 insertions(+) Reviewed-by: Pierrick Bouvier