On Tue, 12 Sep 2023 05:13:19 +0100, zhaoxu <zhaoxu.35@xxxxxxxxxxxxx> wrote: > > > > On 2023/9/4 17:57, Marc Zyngier wrote: > > On Fri, 25 Aug 2023 02:58:11 +0100, > > Xu Zhao <zhaoxu.35@xxxxxxxxxxxxx> wrote: > [...] > >> - unsigned long affinity; > >> - int level0; > >> + u64 aff; > >> - /* > >> - * Split the current VCPU's MPIDR into affinity level 0 and the > >> - * rest as this is what we have to compare against. > >> - */ > >> - affinity = kvm_vcpu_get_mpidr_aff(vcpu); > >> - level0 = MPIDR_AFFINITY_LEVEL(affinity, 0); > >> - affinity &= ~MPIDR_LEVEL_MASK; > >> + /* aff3 - aff1 */ > >> + aff = (((reg) & ICC_SGI1R_AFFINITY_3_MASK) >> ICC_SGI1R_AFFINITY_3_SHIFT) << 16 | > >> + (((reg) & ICC_SGI1R_AFFINITY_2_MASK) >> ICC_SGI1R_AFFINITY_2_SHIFT) << 8 | > >> + (((reg) & ICC_SGI1R_AFFINITY_1_MASK) >> ICC_SGI1R_AFFINITY_1_SHIFT); > > > > Here, you assume that you can directly map a vcpu index to an > > affinity. It would be awesome if that was the case. However, this is > > only valid at reset time, and userspace is perfectly allowed to change > > this mapping by writing to the vcpu's MPIDR_EL1. > > > > So this won't work at all if userspace wants to set its own specific > > CPU numbering. > > > > M. > > > Hi Marc, > > Yes, i don't think too much about userspace can change MPIDR value, I > checked the source code of qemu, qemu create vcpu sequentially, so in > this case, vcpu_id is equivalent to vcpu_idx which means vcpu_id > represents the position in vcpu array. The problem is that this is only a convention, and userspace is totally free to use vcpu_id in a different way. Note that we have other bugs in the KVM code that treat them interchangeably, but I'm trying to fix that. > These days, I'm still thinking about whether it is because of the > content related to future vcpu hot-plug feature that vcpu_id can be > modified, but now it seems that's not entirely the case. There are 3 levels of identification: - vcpu_idx, which is an internal KVM index that userspace is not suppose to rely on (or know) - vcpu_id, which is provided by userspace as a CPU number, and from which we derive the default MPIDR_EL1 value. This is used all over the code to identify a CPU from userspace. - MPIDR_EL1, which is how the architecture identifies a CPU. For CPU hotplug, I expect usespace to set vcpu_id and MPIDR_EL1 as it sees fit, knowing that the vpcu must have been allocated upfront (and vcpu_idx set). > I have read your latest patch and have been deeply inspired, and > Thanks for agreeing with this issue. No worries. I'd appreciate you testing it and reporting whether this matches the results you are observing with your own patch. Thanks, M. -- Without deviation from the norm, progress is not possible.
