Hi Alex, > > > I started reviewing the series, but I ended up being very confused, see > > > below. > > > > > > On Tue, Nov 16, 2021 at 10:43:30PM -0800, Reiji Watanabe wrote: > > > > In KVM/arm64, values of ID registers for a guest are mostly same as > > > > its host's values except for bits for feature that KVM doesn't support > > > > and for opt-in features that userspace didn't configure. Userspace > > > > can use KVM_SET_ONE_REG to a set ID register value, but it fails > > > > if userspace attempts to modify the register value. > > > > > > > > This patch series adds support to allow userspace to modify a value of > > > > ID registers (as long as KVM can support features that are indicated > > > > in the registers) so userspace can have more control of configuring > > > > and unconfiguring features for guests. > > > > > > What not use VCPU features? Isn't that why the field > > > kvm_vcpu_init->features exists in the first place? This cover letter does > > > nothing to explaing why any changes are needed. > > > > > > Do you require finer grained control over certain feature that you cannot > > > get with the 32 * 7 = 224 feature flag bits from kvm_vcpu_init? Does using > > > the ID registers simplify certain aspects of the implementation? > > > > Since some features are not binary in nature (e.g. AA64DFR0_EL1.BRPs > > fields indicate number of breakpoints minus 1), using > > kvm_vcpu_init->features to configure such features is inconvenient. > > I see, this makes a lot of sense and this looks like a nice solution to > that problem. > > > > > One of the reasons why we want the finer grained control is that > > we want to expose a uniform set/level of features for a group of > > guests on systems with different ARM CPUs. > > So here you are talking specifically about KVM not checking that all VCPUs > have the same feature bits set in vcpu->arch.features, which makes it > possible for userspace to set different features for different VCPUs, > right? Yes, that is correct. For features that can be configured by KVM_ARM_VCPU_INIT, userspace can configure different features for different vCPUs by using KVM_ARM_VCPU_INIT as before. Thanks, Reiji _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
