On Mon, Jul 31, 2023 at 04:07:58PM +0800, Like Xu wrote: > From: Like Xu <likexu@xxxxxxxxxxx> > > Add kvm->arch.user_changed_tsc to avoid synchronizing user changes to > the TSC with the KVM-initiated change in kvm_arch_vcpu_postcreate() by > conditioning this mess on userspace having written the TSC at least > once already. > > Here lies UAPI baggage: user-initiated TSC write with a small delta > (1 second) of virtual cycle time against real time is interpreted as an > attempt to synchronize the CPU. In such a scenario, the vcpu's tsc_offset > is not configured as expected, resulting in significant guest service > response latency, which is observed in our production environment. The changelog reads really weird, because it is taken out of context when it isn't a comment over the affected code. Furthermore, 'our production environment' is a complete black box to the rest of the community, it would be helpful spelling out exactly what the use case is. Suggested changelog: KVM interprets writes to the TSC with values within 1 second of each other as an attempt to synchronize the TSC for all vCPUs in the VM, and uses a common offset for all vCPUs in a VM. For brevity's sake let's just ignore what happens on systems with an unstable TSC. While this may seem odd, it is imperative for VM save/restore, as VMMs such as QEMU have long resorted to saving the TSCs (by value) from all vCPUs in the VM at approximately the same time. Of course, it is impossible to synchronize all the vCPU ioctls to capture the exact instant in time, hence KVM fudges it a bit on the restore side. This has been useful for the 'typical' VM lifecycle, where in all likelihood the VM goes through save/restore a considerable amount of time after VM creation. Nonetheless, there are some use cases that need to restore a VM snapshot that was created very shortly after boot (<1 second). Unfortunately the TSC sync code makes no distinction between kernel and user-initiated writes, which leads to the target VM synchronizing on the TSC offset from creation instead of the user-intended value. Avoid synchronizing user-initiated changes to the guest TSC with the KVM initiated change in kvm_arch_vcpu_postcreate() by conditioning the logic on userspace having written the TSC at least once. I'll also note that the whole value-based TSC sync scheme is in desperate need of testing. -- Thanks, Oliver
