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[2001:8b0:1d0::2]) by smtp.gmail.com with ESMTPSA id u7-20020a05600c138700b0040d62f97e3csm15328228wmf.10.2024.01.09.09.08.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 09 Jan 2024 09:08:35 -0800 (PST) From: Peter Maydell To: qemu-arm@nongnu.org, qemu-devel@nongnu.org Subject: [PATCH] docs/system/arm/virt.rst: Improve 'highmem' option docs Date: Tue, 9 Jan 2024 17:08:34 +0000 Message-Id: <20240109170834.1387457-1-peter.maydell@linaro.org> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 Received-SPF: pass client-ip=2a00:1450:4864:20::32b; envelope-from=peter.maydell@linaro.org; helo=mail-wm1-x32b.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_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01, T_SPF_HELO_TEMPERROR=0.01 autolearn=unavailable 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+patch=linaro.org@nongnu.org Sender: qemu-devel-bounces+patch=linaro.org@nongnu.org Improve the 'highmem' option docs to note that by default we assume that a 32-bit kernel on an LPAE-capable CPU has LPAE enabled, and what the consequences are. Signed-off-by: Peter Maydell Reviewed-by: Alex Bennée Reviewed-by: Eric Auger --- docs/system/arm/virt.rst | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst index 7c4c80180c6..c245c52b7ac 100644 --- a/docs/system/arm/virt.rst +++ b/docs/system/arm/virt.rst @@ -96,7 +96,13 @@ mte highmem Set ``on``/``off`` to enable/disable placing devices and RAM in physical address space above 32 bits. The default is ``on`` for machine types - later than ``virt-2.12``. + later than ``virt-2.12`` when the CPU supports an address space + bigger than 32 bits (i.e. 64-bit CPUs, and 32-bit CPUs with the + Large Physical Address Extension (LPAE) feature). If you want to + boot a 32-bit kernel which does not have ``CONFIG_LPAE`` enabled on + a CPU type which implements LPAE, you will need to manually set + this to ``off``; otherwise some devices, such as the PCI controller, + will not be accessible. compact-highmem Set ``on``/``off`` to enable/disable the compact layout for high memory regions.