Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

[Avi Kivity <avi@redhat.com>]

On 10/12/2009 08:20 PM, Jeremy Fitzhardinge wrote:
> On 10/10/09 11:10, Avi Kivity wrote:
>    
>> On 10/10/2009 02:24 AM, Jeremy Fitzhardinge wrote:
>>      
>>> On 10/07/09 03:25, Avi Kivity wrote:
>>>
>>>        
>>>> def try_pvclock_vtime():
>>>>     tsc, p0 = rdtscp()
>>>>     v0 = pvclock[p0].version
>>>>     tsc, p = rdtscp()
>>>>     t = pvclock_time(pvclock[p], tsc)
>>>>     if p != p0 or pvclock[p].version != v0:
>>>>        raise Exception("Processor or timebased change under our feet")
>>>>     return t
>>>>          
> There's a second problem:  If the time_info gets updated between the
> first rdtscp and the first version fetch, then we won't have a
> consistent tsc,time_info pair.  You could check if tsc_timestamp is>
> tsc, but that won't necessarily work on save/restore/migrate.
>    

Good catch.  Doesn't that invalidate rdtscp based vgettimeofday on 
non-virt as well (assuming p == cpu)?

> I suppose that works if you assume that:
>
>     1. every task->vcpu migration is associated with a hv/guest context
>        switch, and
>     2. every hv/guest context switch is a write barrier
>
> I guess 2 is a given, but I can at least imagine cases where 1 might not
> be true.  Maybe.  It all seems very subtle.
>    

What is 1 exactly?  task switching to another vcpu?  that doesn't incur 
hypervisor involvement.  vcpu moving to another cpu?  That does.

> And I don't really see a gain.  You avoid maintaining a second version
> number, but at the cost of two rdtscps.  In my measurements, the whole
> vsyscall takes around 100ns to run, and a single rdtsc takes about 30,
> so 30% of total.  Unlike rdtsc, rdtscp is documented as being ordered in
> the instruction stream, and so will take at least as long; two of them
> will completely blow the vsyscall execution time.
>    

I agree, let's stick with the rdtscpless implementation.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.