[PATCH v5 41/51] x86/kvmclock: Refactor handling of PVCLOCK_TSC_STABLE_BIT during kvmclock_init()
From: Sean Christopherson <seanjc@google.com>
Date: 2026-07-01 19:33:17
Also in:
kvm, linux-doc, linux-hyperv, lkml, virtualization, xen-devel
Subsystem:
kvm paravirt (kvm/paravirt), the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Paolo Bonzini, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
Clean up the setting of PVCLOCK_TSC_STABLE_BIT during kvmclock init to make it somewhat obvious that pvclock_read_flags() must be called *after* pvclock_set_flags(). Note, in theory, a different PV clock could have set PVCLOCK_TSC_STABLE_BIT in the supported flags, i.e. reading flags only if KVM_FEATURE_CLOCKSOURCE_STABLE_BIT is set could very, very theoretically result in a change in behavior. In practice, the kernel only supports a single PV clock. Reviewed-by: David Woodhouse <redacted> Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kernel/kvmclock.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 5220d205abc7..61d4d943fe74 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c@@ -327,7 +327,7 @@ static __init void kvm_sched_clock_init(bool stable) void __init kvmclock_init(bool prefer_tsc) { - u8 flags; + bool stable = false; if (!kvm_para_available() || !kvmclock) return;
@@ -354,11 +354,18 @@ void __init kvmclock_init(bool prefer_tsc) kvm_register_clock("primary cpu clock"); pvclock_set_pvti_cpu0_va(hv_clock_boot); - if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT)) + if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT)) { pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT); - flags = pvclock_read_flags(&hv_clock_boot[0].pvti); - kvm_sched_clock_init(flags & PVCLOCK_TSC_STABLE_BIT); + /* + * Check if the clock is stable *after* marking TSC_STABLE as a + * valid flag. + */ + stable = pvclock_read_flags(&hv_clock_boot[0].pvti) & + PVCLOCK_TSC_STABLE_BIT; + } + + kvm_sched_clock_init(stable); if (!x86_init.hyper.get_tsc_khz) x86_init.hyper.get_tsc_khz = kvmclock_get_tsc_khz;
--
2.55.0.rc0.799.gd6f94ed593-goog