From 1154d84dcc5f46e83db94281d071775819dd8884 Mon Sep 17 00:00:00 2001 From: Eduardo Habkost Date: Mon, 3 Nov 2014 15:45:34 -0200 Subject: kvmclock: Add comment explaining why we need cpu_clean_all_dirty() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Try to explain why commit 317b0a6d8ba44e9bf8f9c3dbd776c4536843d82c needed a cpu_clean_all_dirty() call just after calling cpu_synchronize_all_states(). Signed-off-by: Eduardo Habkost Cc: Andrey Korolyov Cc: Marcin Gibuła Cc: Marcelo Tosatti Cc: Paolo Bonzini Signed-off-by: Paolo Bonzini --- hw/i386/kvm/clock.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'hw') diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c index 1ac60d6cdd..58be2bda27 100644 --- a/hw/i386/kvm/clock.c +++ b/hw/i386/kvm/clock.c @@ -127,7 +127,21 @@ static void kvmclock_vm_state_change(void *opaque, int running, } cpu_synchronize_all_states(); + /* In theory, the cpu_synchronize_all_states() call above wouldn't + * affect the rest of the code, as the VCPU state inside CPUState + * is supposed to always match the VCPU state on the kernel side. + * + * In practice, calling cpu_synchronize_state() too soon will load the + * kernel-side APIC state into X86CPU.apic_state too early, APIC state + * won't be reloaded later because CPUState.vcpu_dirty==true, and + * outdated APIC state may be migrated to another host. + * + * The real fix would be to make sure outdated APIC state is read + * from the kernel again when necessary. While this is not fixed, we + * need the cpu_clean_all_dirty() call below. + */ cpu_clean_all_dirty(); + ret = kvm_vm_ioctl(kvm_state, KVM_GET_CLOCK, &data); if (ret < 0) { fprintf(stderr, "KVM_GET_CLOCK failed: %s\n", strerror(ret)); -- cgit v1.2.3