Re: [PATCH] powerpc/time: use get_tb instead of get_vtb in running_clock
From: Benjamin Herrenschmidt
Date: Wed Jul 12 2017 - 18:46:00 EST
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 .
> 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.
>  http://git.qemu.org/?p=qemu.git;a=commit;h=42043e4f1
> Signed-off-by: Jia He <hejianet@xxxxxxxxx>
> 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) &&
> - 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.