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 14:38
> 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 14:09, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> From: Paul Durrant
> >> Sent: 06 March 2019 13:06
> >>
> >> > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> > Sent: 06 March 2019 12:57
> >> > 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:
> >> > >> > +{
> >> > >> > +    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.
> >
> > Both those examples do the multiplication by inline asm (presumably to avoid
> > truncation). Is that what you'd prefer?
> 
> That would imo be best, not the least because of making us independent
> of possible issues with 128-bit arithmetic on older compilers.
> 

Ok, I think I have figured out the necessary runes so I'll do that.

  Paul

> Jan
> 


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