Re: [Xen-devel] [PATCH] viridian: unify time sources

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 13 June 2019 15:47
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Roger Pau Monne 
> <roger.pau@xxxxxxxxxx>; xen-devel <xen-
> devel@xxxxxxxxxxxxxxxxxxxx>; WeiLiu <wl@xxxxxxx>
> Subject: Re: [PATCH] viridian: unify time sources
> 
> >>> On 11.06.19 at 19:01, <paul.durrant@xxxxxxxxxx> wrote:
> > @@ -77,6 +76,7 @@ static void update_reference_tsc(struct domain *d, bool 
> > initialize)
> >       * ticks per 100ns shifted left by 64.
> >       */
> >      p->TscScale = ((10000ul << 32) / d->arch.tsc_khz) << 32;
> > +    p->TscOffset = trc->off;
> >      smp_wmb();
> 
> Even having written the description it's not immediately obvious
> to me why this is an okay change to make, not the least when
> thinking about a guest migrating in from an older version. At the
> very least don't you think the comment above may want slightly
> extending?

I guess I could add a flag in the migration stream but, as described in the 
commit comment, the guest will see a jump if the page happens to get 
invalidated, so it's always a risk. I'll extend the comment as you suggest.

> 
> Everything else looks plausible to me.

Ok, thanks.

  Paul

> 
> Jan
> 


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