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Fri, 10 Mar 2023 12:29:21 +0000 Date: Fri, 10 Mar 2023 12:29:20 +0000 Message-ID: <86sfecyelb.wl-maz@kernel.org> From: Marc Zyngier To: Mark Brown Cc: Joey Gouly , linux-arm-kernel@lists.infradead.org, nd@arm.com, catalin.marinas@arm.com, james.morse@arm.com, mark.rutland@arm.com, oliver.upton@linux.dev, suzuki.poulose@arm.com, will@kernel.org, yuzenghui@huawei.com Subject: Re: [PATCH v1 11/18] KVM: arm64: expose ID_AA64MMFR3_EL1 to guests In-Reply-To: <319dcb3f-7216-4665-b628-dca564088910@sirena.org.uk> References: <20230309145246.22787-1-joey.gouly@arm.com> <20230309145246.22787-12-joey.gouly@arm.com> <86y1o5yjs7.wl-maz@kernel.org> <319dcb3f-7216-4665-b628-dca564088910@sirena.org.uk> 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/28.2 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: broonie@kernel.org, joey.gouly@arm.com, linux-arm-kernel@lists.infradead.org, nd@arm.com, catalin.marinas@arm.com, james.morse@arm.com, mark.rutland@arm.com, oliver.upton@linux.dev, suzuki.poulose@arm.com, will@kernel.org, yuzenghui@huawei.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230310_042930_522242_B32B41FC X-CRM114-Status: GOOD ( 26.92 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, 09 Mar 2023 17:04:20 +0000, Mark Brown wrote: > > On Thu, Mar 09, 2023 at 04:24:56PM +0000, Marc Zyngier wrote: > > Mark Brown wrote: > > > On Thu, Mar 09, 2023 at 02:52:39PM +0000, Joey Gouly wrote: > > > > > Now that KVM context switches the appropriate registers, expose > > > > ID_AA64MMFR3_EL1 to guests to allow them to use the new features. > > > > > > Should we be adding new vCPU features given that there's new > > > architectural state here? > > > What do we gain by that? AFAICT, it only makes the UAPI more complex. > > And to be honest, *any* new feature added to KVM results in new > > architectural state. Are we going to add more and more of these ad > > nauseam? It doesn't scale. All we need to ensure at this stage is that > > you cannot migrate a VM that has seen this to a host that doesn't have > > the feature. > > That was a genuine question prompted by how we handle pointer auth - > that seemed to be in a similar situation but has a flag. With something > like SVE which causes the V registers to be replaced by the Z registers > in the view offered to VMMs it's more obvious. > > > And if anything, this sort of selection should be defined by writing > > to the ID_AA64MMFR3_EL1 register from userspace and let the whole > > thing be driven by it. See Jing's current effort at [1]. > > Yes, I've seen that and it does seem a lot more logical - I was always > confused by why we couldn't do that normally. Because we made the mistake initially and nobody cared enough to fix it until now? The KVM UAPI is a never ending train wreck... M. -- Without deviation from the norm, progress is not possible. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel