Re: [Xen-devel] [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

[David Vrabel <dvrabel@cantab.net>]

On 23/12/2014 00:39, Andy Lutomirski wrote:
> The pvclock vdso code was too abstracted to understand easily and
> excessively paranoid.  Simplify it for a huge speedup.
>
> This opens the door for additional simplifications, as the vdso no
> longer accesses the pvti for any vcpu other than vcpu 0.
>
> Before, vclock_gettime using kvm-clock took about 64ns on my machine.
> With this change, it takes 19ns, which is almost as fast as the pure TSC
> implementation.

Xen guests don't use any of this at the moment, and I don't think this 
change would prevent us from using it in the future, so:

Acked-by: David Vrabel <david.vrabel@citrix.com>

But see some additional comments below.

> --- a/arch/x86/vdso/vclock_gettime.c
> +++ b/arch/x86/vdso/vclock_gettime.c
> @@ -78,47 +78,59 @@ static notrace const struct pvclock_vsyscall_time_info *get_pvti(int cpu)
>
>   static notrace cycle_t vread_pvclock(int *mode)
>   {
> -	const struct pvclock_vsyscall_time_info *pvti;
> +	const struct pvclock_vcpu_time_info *pvti = &get_pvti(0)->pvti;

Xen updates pvti when scheduling a VCPU.  Using 0 here requires that 
VCPU 0 has been recently scheduled by Xen.  Perhaps using the current 
CPU here would be better?  It doesn't matter if the task is subsequently 
moved to a different CPU before using pvti.

> +	 * Note: The kernel and hypervisor must guarantee that cpu ID
> +	 * number maps 1:1 to per-CPU pvclock time info.
> +	 *
> +	 * Because the hypervisor is entirely unaware of guest userspace
> +	 * preemption, it cannot guarantee that per-CPU pvclock time
> +	 * info is updated if the underlying CPU changes or that that
> +	 * version is increased whenever underlying CPU changes.
> +	 *
> +	 * On KVM, we are guaranteed that pvti updates for any vCPU are
> +	 * atomic as seen by *all* vCPUs.  This is an even stronger
> +	 * guarantee than we get with a normal seqlock.
>   	 *
> +	 * On Xen, we don't appear to have that guarantee, but Xen still
> +	 * supplies a valid seqlock using the version field.
> +
> +	 * We only do pvclock vdso timing at all if
> +	 * PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
> +	 * mean that all vCPUs have matching pvti and that the TSC is
> +	 * synced, so we can just look at vCPU 0's pvti.

I think this is a much stronger requirement than you actually need.

You only require:

- the system time (pvti->system_time) for all pvti's is synchronized; and
- TSC is synchronized; and
- the pvti has been updated sufficiently recently (so the error in the 
result is within acceptable margins).

Can you add documentation to arch/x86/include/asm/pvclock-abi.h to 
describe what properties PVCLOCK_TSC_STABLE_BIT guarantees?

David