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Re: [Xen-devel] [PATCH v8 10/11] viridian: add implementation of synthetic timers
- To: 'Jan Beulich' <JBeulich@xxxxxxxx>
- From: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
- Date: Mon, 18 Mar 2019 16:26:05 +0000
- Accept-language: en-GB, en-US
- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, "Tim \(Xen.org\)" <tim@xxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 18 Mar 2019 16:26:13 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [Xen-devel] [PATCH v8 10/11] viridian: add implementation of synthetic timers
> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 18 March 2019 16:23
> 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: [Xen-devel] [PATCH v8 10/11] viridian: add implementation of
> synthetic timers
>
> >>> On 18.03.19 at 16:46, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> > > + {
> >> > > + expiration = vs->count;
> >> > > + if ( expiration - now <= 0 )
> >> > > + {
> >> > > + vs->expiration = expiration;
> >> > > + stimer_expire(vs);
> >> >
> >> > Aren't you introducing a risk for races by calling the timer function
> >> > directly from here? start_timer(), after all, gets called from quite a
> >> > few places.
> >>
> >> In practice I don't think there should be any problematic race, but I'll
> >> check again.
> >
> > I think the 'periodic' name might be confusing things... The Xen timers are
> > all single-shot, it's just that start_stimer() is re-called after a
> > successful poll if the viridian timer is configured to be periodic. So I
> > don't think there is case where the underlying Xen timer could actually be
> > running when we enter start_stimer().
>
> One of the callers of the function is the WRMSR handler. Why would
> it be guaranteed that the timer isn't active when such a WRMSR
> occurs?
It's not guaranteed on entry, but the WRMSR handler always calls stop_stimer()
before calling start_stimer() which AFAICT should guarantee the timer is not
running when start_stimer() is called.
Paul
> Of course, this alone is not enough for there to be a problem,
> as we're fine as long as the guest can only harm itself. But I don't
> think it goes without saying that there's no issue here; having looked
> a 2nd time just now, I think I agree though that there's no risk for our
> own health here.
>
> Jan
>
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