Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: - [High] kvm_sched_clock_read is registered as the system sched_clock before its required offset (kvm_sched_clock_offset) is initialized. --
commit b6e429e4990c00d845488d46e8dc4132f3a2fd8b Author: Sean Christopherson <[email protected]> x86/paravirt: Plumb a return code into __paravirt_set_sched_clock() This commit adds a return code to __paravirt_set_sched_clock() to allow the kernel to reject attempts to use a PV sched_clock. It updates the caller in kvmclock to check the return value and abort initialization if needed. > diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c > index d3bb281c0805d..9b3d1ed1a96dc 100644 > --- a/arch/x86/kernel/kvmclock.c > +++ b/arch/x86/kernel/kvmclock.c [ ... ] > @@ -338,10 +338,12 @@ static int kvmclock_setup_percpu(unsigned int cpu) > > static __init void kvm_sched_clock_init(bool stable) > { > + if (__paravirt_set_sched_clock(kvm_sched_clock_read, stable, > + kvm_save_sched_clock_state, > + kvm_restore_sched_clock_state)) > + return; > + > kvm_sched_clock_offset = kvm_clock_read(); Could this cause a massive backward jump in system time if an NMI or early exception calls sched_clock() right after registration? __paravirt_set_sched_clock() uses static_call_update() to globally publish kvm_sched_clock_read() as the live system sched_clock. Since the initialization of kvm_sched_clock_offset is now delayed until after this registration completes, any early caller will execute kvm_sched_clock_read() with a zero offset, returning a huge absolute raw clock value. When the code resumes, it sets the offset to the current raw clock value. Subsequent sched_clock() calls will abruptly subtract this large offset, causing time to jump backwards and violating monotonic clock guarantees. Would it be safer to initialize kvm_sched_clock_offset before calling __paravirt_set_sched_clock(), since calculating the offset early has no side effects even if the registration is subsequently rejected? > - __paravirt_set_sched_clock(kvm_sched_clock_read, stable, > - kvm_save_sched_clock_state, > - kvm_restore_sched_clock_state); > kvmclock_is_sched_clock = true; > > /* -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=29
