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From: Oliver Upton To: "Russell King (Oracle)" Cc: Marc Zyngier , Catalin Marinas , Will Deacon , James Morse , Suzuki K Poulose , Zenghui Yu , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev Subject: Re: [PATCH RFC] KVM: arm64: allow ID_MMFR4_EL1 to be writable Message-ID: References: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240501_115935_353136_DEC7A38E X-CRM114-Status: GOOD ( 32.58 ) 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 Wed, May 01, 2024 at 07:08:05PM +0100, Russell King (Oracle) wrote: > On Wed, May 01, 2024 at 05:57:20PM +0000, Oliver Upton wrote: > > Hi Russell, > > > > On Wed, May 01, 2024 at 06:06:51PM +0100, Russell King (Oracle) wrote: > > > Between 5.4 and 5.15, the guests view of HPDS, CnP, XNX and AC2 > > > changed their value on the same Neoverse N1 r3p1 hardware which makes > > > migrating between these kernels on the host problematical. > > > > It'd be helpful to expand a bit more on how these fields changed, better > > yet if we can blame it back to a commit. I'm guessing the only direction > > of migration you care about is old -> new then? > > Yes. For MMFR4_EL1, we see 0 with our 5.4 based kernel, and 0x21110 > with our 5.15 kernel. I've been looking at tracking down which commit > is responsible but I've come up with nothing that fits. > > The only change I can see is the FTR definition for MMFR4, but this > always included 4:7 (AC2) which changed 0 -> 1. So... no idea what > commit caused the change. > > There are a load of other registers that we need sorting, but this > is just a test forray into attempting to solve this. Got it, let me see if I can find it then. Do share that list of problematic registers when you have it, hopefully this isn't the tip of the iceberg... > > > > > We already permit changing HPDS in AA64MMFR1_EL1 and CnP in > > > AA64MMFR2_EL1. We also allow LSM as we allow that in AA64MMFR2_EL1, > > > so this patch includes support for that even though it isn't required. > > > > > > Discussing with Marc Zygnier, AC2 should also be fine to be writable, > > > > typo: Zyngier > > > > > even though we don't inject an UNDEF if the guest accesses those > > > registers. > > > > > > The only questionable one is XNX, execute-never control distinction, > > > which is also in AA64MMFR1_EL1 but we don't allow to be changed there. > > > > It is quite odd for us to expose this field to non-nested VMs in the > > first place, though I suppose we will apply an additional set of > > restrictions for nested VMs when they come along. > > Yes, it did strike me as odd, since the description seems to imply that > XNX affects EL2, which the VM wouldn't have access to. So I'm not sure > why we don't just force it to zero. Probably because we failed to catch it in the first place and setting to 0 now would be even more UAPI breakage. Meh :-/ I don't see any immediate issues with the patch, especially since it is fixing a genuine UAPI breakage in KVM. -- Thanks, Oliver _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel