On Wed, 2017-07-12 at 23:01 +0800, Jia He wrote: > Virtual time base(vtb) is a register which increases only in guest. > Any exit from guest to host will stop the vtb(saved and restored by kvm). > But if there is an IO causes guest exits to host, the guest's watchdog > (watchdog_timer_fn -> is_softlockup -> get_timestamp -> running_clock) > needs to also include the time elapsed in host. get_vtb is not correct in > this case. > > Also, the TB_OFFSET is well saved and restored by qemu after commit [1]. > So we can use get_tb here.
That completely defeats the purpose here... This was done specifically to exploit the VTB which doesn't count in hypervisor mode. > > [1] http://git.qemu.org/?p=qemu.git;a=commit;h=42043e4f1 > > Signed-off-by: Jia He <hejia...@gmail.com> > --- > arch/powerpc/kernel/time.c | 7 +++---- > 1 file changed, 3 insertions(+), 4 deletions(-) > > diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c > index fe6f3a2..c542dd3 100644 > --- a/arch/powerpc/kernel/time.c > +++ b/arch/powerpc/kernel/time.c > @@ -695,16 +695,15 @@ notrace unsigned long long sched_clock(void) > unsigned long long running_clock(void) > { > /* > - * Don't read the VTB as a host since KVM does not switch in host > - * timebase into the VTB when it takes a guest off the CPU, reading the > - * VTB would result in reading 'last switched out' guest VTB. > + * Use get_tb instead of get_vtb for guest since the TB_OFFSET has been > + * well saved/restored when qemu does suspend/resume. > * > * Host kernels are often compiled with CONFIG_PPC_PSERIES checked, it > * would be unsafe to rely only on the #ifdef above. > */ > if (firmware_has_feature(FW_FEATURE_LPAR) && > cpu_has_feature(CPU_FTR_ARCH_207S)) > - return mulhdu(get_vtb() - boot_tb, tb_to_ns_scale) << > tb_to_ns_shift; > + return mulhdu(get_tb() - boot_tb, tb_to_ns_scale) << > tb_to_ns_shift; > > /* > * This is a next best approximation without a VTB.
