Re: [Xen-devel] [PATCH v3 8/9] viridian: add implementation of synthetic timers

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 06 March 2019 12:57
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx>; Andrew Cooper 
> <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap
> <George.Dunlap@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Roger Pau 
> Monne
> <roger.pau@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; Stefano Stabellini 
> <sstabellini@xxxxxxxxxx>;
> xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>; Konrad Rzeszutek Wilk 
> <konrad.wilk@xxxxxxxxxx>; Tim
> (Xen.org) <tim@xxxxxxx>
> Subject: RE: [PATCH v3 8/9] viridian: add implementation of synthetic timers
> 
> >>> On 06.03.19 at 12:23, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: 25 February 2019 14:54
> >>
> >> >>> On 31.01.19 at 11:47, <paul.durrant@xxxxxxxxxx> wrote:
> >> > @@ -118,14 +119,237 @@ static int64_t time_ref_count(struct domain *d)
> >> >      return raw_trc_val(d) + trc->off;
> >> >  }
> >> >
> >> > +static int64_t time_now(struct domain *d)
> >>
> >> Why would this return a signed value? And can't the function
> >> parameter be const?
> >
> > The function parameter can be const, but I think the result needs to be
> > signed for the missed ticks logic in start_timer() to work correctly.
> 
> If something requires signed arithmetic, then this should be enforced
> there, not by the return type of an otherwise sufficiently generic
> function. Then again NOW() also produces a signed value ...
> 
> >> > +{
> >> > +    const struct viridian_page *rt = 
> >> > &d->arch.hvm.viridian->reference_tsc;
> >> > +    HV_REFERENCE_TSC_PAGE *p = rt->ptr;
> >> > +    uint32_t start, end;
> >> > +    __int128_t tsc;
> >> > +    __int128_t scale;
> >>
> >> I don't think you need both of them be 128 bits wide. I also don't
> >> see why either would want to be of a signed type.
> >
> > The spec says (as in the comment below):
> >
> > "The partition reference time is computed by the following formula:
> >
> > ReferenceTime = ((VirtualTsc * TscScale) >> 64) + TscOffset
> >
> > The multiplication is a 64 bit multiplication, which results in a 128 bit
> > number which is then shifted 64 times to the right to obtain the high 64
> > bits.TscScale"
> 
> Well, yes, you want a 128-bit result. But for that you don't need to
> multiply 128-bit quantities. See e.g. our own scale_delta() or
> hvm_scale_tsc().

Testing showed that by not casting first things were broken. I assumed this was 
because the result of the multiplication was being truncated to 64-bits before 
assignment, but I can check the generated code. I'll also have a look at the 
examples you cite.

  Paul


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