Re: [PATCH v3 02/18] x86/msr/kvm: Remove vget_cycles()

Paolo Bonzini
2015-06-17T13:34:27Z

On 17/06/2015 02:35, Andy Lutomirski wrote:
> The only caller was kvm's read_tsc. The only difference between
> vget_cycles and native_read_tsc was that vget_cycles returned zero
> instead of crashing on TSC-less systems. KVM's already checks
> vclock_mode before calling that function, so the extra check is
> unnecessary.

Or more simply, KVM (host-side) requires the TSC to exist.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

> (Off-topic, but the whole KVM clock host implementation is gross.
> IMO it should be rewritten.)
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
> arch/x86/include/asm/tsc.h | 13 -------------
> arch/x86/kvm/x86.c | 2 +-
> 2 files changed, 1 insertion(+), 14 deletions(-)
>
> diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
> index fd11128faf25..3da1cc1218ac 100644
> --- a/arch/x86/include/asm/tsc.h
> +++ b/arch/x86/include/asm/tsc.h
> @@ -32,19 +32,6 @@ static inline cycles_t get_cycles(void)
> return ret;
> }
>
> -static __always_inline cycles_t vget_cycles(void)
> -{
> - /*
> - * We only do VDSOs on TSC capable CPUs, so this shouldn't
> - * access boot_cpu_data (which is not VDSO-safe):
> - */
> -#ifndef CONFIG_X86_TSC
> - if (!cpu_has_tsc)
> - return 0;
> -#endif
> - return (cycles_t)native_read_tsc();
> -}
> -
> extern void tsc_init(void);
> extern void mark_tsc_unstable(char *reason);
> extern int unsynchronized_tsc(void);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 26eaeb522cab..c26faf408bce 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1430,7 +1430,7 @@ static cycle_t read_tsc(void)
> * but no one has ever seen it happen.
> */
> rdtsc_barrier();
> - ret = (cycle_t)vget_cycles();
> + ret = (cycle_t)native_read_tsc();
>
> last = pvclock_gtod_data.clock.cycle_last;
>
>