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Fri, 23 Oct 2026 08:31:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792744295; bh=NShwj3uySdZgYCjF4f6Gih+68kU6l/C0njQVpdn1AzA=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=SVHaoBALR7WJ71D1liXzObFE+vCHKRrdo3c5LpI+6iVskBn02BK/NlEMU4dxHSTbT eUvJnwQnFMYfOQJ9CXtpGs7sScqi38Dj4CBHZc6Cr11rf/x9uks0Pu6VteITazLQEX Pqa3bcJ2nweoJarZTzWpyrLy4uhH9MdZIQEX26nN6be9wuMtTtICUzToiY2qDin5dR bu52nrKzoUMNX7stcph1UIvAmwda1E7V2E5hPkydNkGAv1hlbTZW4jPQh3rHmJle9d SdOlY3O7f6dmcnueb2RxBBmQBlxwANA4rT/WMl733fj//RUC8l8sTYbWYYrTuGpo9U 4eDXUFodChlNQ== Received: from sofa.misterjones.org ([185.219.108.64] helo=goblin-girl.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.98.2) (envelope-from ) id 1xKAgW-00000006NxD-3HaW; Fri, 23 Oct 2026 08:31:32 +0000 Date: Fri, 23 Oct 2026 09:31:32 +0100 Message-ID: <86fqxwzul7.wl-maz@kernel.org> From: Marc Zyngier To: Fuad Tabba Cc: Oliver Upton , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, Joey Gouly , Suzuki K Poulose , Zenghui Yu , Steffen Eiden , Catalin Marinas , Will Deacon , Mark Rutland , Yicong Yang , Zhou Wang , Arnd Bergmann , Fuad Tabba , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] KVM: arm64: Hide FEAT_LS64_ACCDATA from guests In-Reply-To: <20261023070110.1833446-2-fuad.tabba@linux.dev> References: <20261023070110.1833446-1-fuad.tabba@linux.dev> <20261023070110.1833446-2-fuad.tabba@linux.dev> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/30.1 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) X-Archived-By: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: fuad.tabba@linux.dev, oupton@kernel.org, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, joey.gouly@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, seiden@linux.ibm.com, catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com, yangyicong@hisilicon.com, wangzhou1@hisilicon.com, arnd@arndb.de, tabba@google.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-6.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Fri, 23 Oct 2026 08:01:09 +0100, Fuad Tabba wrote: > > Since commit 58ce78667a641 ("arm64: Add support for FEAT_{LS64, > LS64_V}"), a guest sees the host's ID_AA64ISAR1_EL1.LS64, but KVM only > enables the FEAT_LS64 and FEAT_LS64_V instructions in HCRX_EL2. On a > host with FEAT_LS64_ACCDATA, a guest's ST64BV0 therefore traps and hits > the WARN_ON_ONCE() in handle_other(). ACCDATA_EL1 isn't trapped or > context-switched either, so every guest on that CPU shares it. > > Rather than support FEAT_LS64_ACCDATA, which requires ACCDATA_EL1 to be > context-switched, cap the field at LS64_V. The guest's ACCDATA_EL1 > accesses then UNDEF. This hides FEAT_LS64WB too, as it implies > FEAT_LS64_ACCDATA. Why settle for the least capable option? Context switching ACCDATA_EL1 is easy enough, can be gated on guest features, and avoid the userspace horror story. I'd rather see that. M. -- Without deviation from the norm, progress is not possible.