Hi Marc, On Mon, May 29, 2023 at 02:39:28PM +0100, Marc Zyngier wrote: > On Sat, 27 May 2023 05:02:32 +0100, > Reiji Watanabe <reijiw@xxxxxxxxxx> wrote: > > > > This series fixes issues with PMUVer handling for a guest with > > PMU configured on heterogeneous PMU systems. > > Specifically, it addresses the following two issues. > > > > [A] The default value of ID_AA64DFR0_EL1.PMUVer of the vCPU is set > > to its sanitized value. This could be inappropriate on > > heterogeneous PMU systems, as arm64_ftr_bits for PMUVer is defined > > as FTR_EXACT with safe_val == 0 (when ID_AA64DFR0_EL1.PMUVer of all > > PEs on the host is not uniform, the sanitized value will be 0). > > Why is this a problem? The CPUs don't implement the same version of > the architecture, we don't get a PMU. Why should we try to do anything > better? I really don't think we should go out or out way and make the > code more complicated for something that doesn't really exist. Even when the CPUs don't implement the same version of the architecture, if one of them implement PMUv3, KVM advertises KVM_CAP_ARM_PMU_V3, and allows userspace to configure PMU (KVM_ARM_VCPU_PMU_V3) for vCPUs. In this case, although KVM provides PMU emulations for the guest, the guest's ID_AA64DFR0_EL1.PMUVer will be zero. Also, KVM_SET_ONE_REG for ID_AA64DFR0_EL1 will never work for vCPUs with PMU configured on such systems (since KVM also doesn't allow userspace to set the PMUVer to 0 for the vCPUs with PMU configured). I would think either ID_AA64DFR0_EL1.PMUVer for the guest should indicate PMUv3, or KVM should not allow userspace to configure PMU, in this case. This series is a fix for the former, mainly to keep the current behavior of KVM_CAP_ARM_PMU_V3 and KVM_ARM_VCPU_PMU_V3 on such systems, since I wasn't sure if such systems don't really exist :) (Also, I plan to implement a similar fix for PMCR_EL0.N on top of those changes) I could make a fix for the latter instead though. What do you think ? Thank you, Reiji > > Or am I missing the problem altogether? > > Thanks, > > M. > > -- > Without deviation from the norm, progress is not possible.
