Re: [Xen-devel] [PATCH v3 1/7] introduce time managment in xtf

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Apr 10, 2018 at 12:32:23PM +0200, Paul Semel wrote:
> On 04/10/2018 12:05 PM, Roger Pau Monné wrote:
> > > > > > > this file is introduce to be able to implement an inter domain
> > > > > > > communication protocol over xenstore. For synchronization 
> > > > > > > purpose, we do
> > > > > > > really want to be able to "control" time
> > > > > > > 
> > > > > > > common/time.c: since_boot_time gets the time in nanoseconds from 
> > > > > > > the
> > > > > > > moment the VM has booted
> > > > > > > 
> > > > > > > Signed-off-by: Paul Semel <phentex@xxxxxxxxx>
> > > > > > > ---
> > > > > > 
> > > > > > This seems to be missing a list of changes between v2 and v3. Please
> > > > > > add such a list when posting new versions.
> > > > > > 
> > > > > > > +uint64_t since_boot_time(void)
> > > > > > > +{
> > > > > > > +    uint64_t tsc;
> > > > > > > +    uint32_t ver1, ver2;
> > > > > > > +    uint64_t system_time;
> > > > > > > +    uint64_t old_tsc;
> > > > > > > +
> > > > > > > +    do
> > > > > > > +    {
> > > > > > > +        do
> > > > > > > +        {
> > > > > > > +            ver1 = 
> > > > > > > ACCESS_ONCE(shared_info.vcpu_info[0].time.version);
> > > > > > > +            smp_rmb();
> > > > > > > +        } while ( (ver1 & 1) == 1 );
> > > > > > > +
> > > > > > > +        system_time = 
> > > > > > > ACCESS_ONCE(shared_info.vcpu_info[0].time.system_time);
> > > > > > > +        old_tsc = 
> > > > > > > ACCESS_ONCE(shared_info.vcpu_info[0].time.tsc_timestamp);
> > > > > > > +        smp_rmb();
> > > > > > > +        ver2 = 
> > > > > > > ACCESS_ONCE(shared_info.vcpu_info[0].time.version);
> > > > > > > +        smp_rmb();
> > > > > > > +    } while ( ver1 != ver2 );
> > > > > > 
> > > > > > This is still overly complicated IMO, and you have not replied to my
> > > > > > question of whether doing the scale_delta below is OK.
> > > > > 
> > > > > About this scale_delta, we discussed with Andrew, and we are going to 
> > > > > use
> > > > > another version of the function as far as I remember. That's why I am 
> > > > > not
> > > > > taking care of it for the moment.
> > > > 
> > > > You should send that version then :).
> > > > 
> > > > > > 
> > > > > > AFAICT uou _cannot_ access any of the vcpu_time_info fields without
> > > > > > checking for the version (in order to avoid reading inconsistent 
> > > > > > data
> > > > > > during an update), yet below you read tsc_to_system_mul and
> > > > > > tsc_shift.
> > > > > > 
> > > > > 
> > > > > I'm sorry, I am probably not getting your point here, because I am 
> > > > > already
> > > > > checking for the version. I was actually checking for the wc_version 
> > > > > too in
> > > > > the first version of those patches, but after chatting with Andrew, It
> > > > > appeared that it was not necessary..
> > > > 
> > > > AFAICT the following should work:
> > > > 
> > > > do
> > > > {
> > > >       ver1 = shared_info.vcpu_info[0].time.version;
> > > >       smp_rmb();
> > > > 
> > > >       system_time = shared_info.vcpu_info[0].time.system_time;
> > > >       tsc_timestamp = shared_info.vcpu_info[0].time.tsc_timestamp;
> > > >       tsc_to_system_mul = 
> > > > shared_info.vcpu_info[0].time.tsc_to_system_mul;
> > > >       tsc_shift = shared_info.vcpu_info[0].time.tsc_shift;
> > > >       tsc = rdtsc_ordered();
> > > >       /* NB: this barrier is probably not needed if rdtsc is 
> > > > serializing. */
> > > >       smp_rmb();
> > > > 
> > > >       ver2 = ACCESS_ONCE(shared_info.vcpu_info[0].time.version);
> > > > } while ( ver2 & 1 || ver1 != ver2 );
> > > > 
> > > 
> > > Just a (probably dumb) question. Why aren't you doing ACCESS_ONCE on every
> > > shared_info field accesses ?
> > > As far as I understand, we need to do this as most as we can to avoid 
> > > having
> > > completely broken data (or security issues). Am I missing something ?
> > 
> > ACCESS_ONCE prevents the reordering of the reads, but here AFAICT we
> > don't really care about the order in which way they are performed as
> > long as they are all done before the read barrier (smp_rmb).
> > 
> > Note that I used ACCESS_ONCE for the last access to the version field.
> > I've done that to prevent the compiler from optimizing the code as:
> > 
> > } while ( shared_info.vcpu_info[0].time.version & 1 ||
> >            ver1 != shared_info.vcpu_info[0].time.version );
> > 
> > Which would be incorrect, since we want to use the same version data
> > for both checks in the while loop condition.
> > 
> 
> Okay, I really thought that it was also used to ensure that the accesses are
> not splitted into multiple instructions (for optimizations because of the
> loop), and thus put us in trouble if the shared memory was modified in
> between.

If the memory is modified in between either (ver2 & 1) == 1 or ver1 !=
ver2 because that's the protocol between the hypervisor and the guest
in order to update vpcu_time_info, so we will discard the read data
and start the loop again.

Roger.

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