Re: [Xen-devel] [PATCH] xen/pciback: Update data filter intersection logic.

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 06/20/2016 04:56 AM, Jan Beulich wrote:
> > > > On 20.06.16 at 00:03, <andrey2805@xxxxxxxxx> wrote:
> > Follow up on http://www.gossamer-threads.com/lists/xen/devel/436000#436000
> > Using http://eli.thegreenplace.net/2008/08/15/intersection-of-1d-segments as
> > reference.
> >
> >             New value
> >         |---------------|
> >
> > f1                            f5
> > |---|                       |-----|
> >        f2             f4
> >      |-----|    f3   |-----|
> >             |-----|
> >
> > Given a new value of the type above, Current logic will not
> > allow applying parts of the new value overlapping with f3 filter.
> > only f2 and f4.
> I remains unclear what f1...f5 are meant to represent here.
>
> > This change allows all 3 types of overlapes to be included.
> > More specifically for passthrough an Industrial Ethernet Interface
> > (Hilscher GmbH CIFX 50E-DP(M/S)) on a HVM DomU running the
> > Xen 4.6 Hypervisor it allows to restore the LATENCY TIMER field
> > given a quirk to allow read/write for that field is already in place.
> > Device driver logic is such that the entire confspace  is
> > written in 4 byte chunks s.t. LATENCY_TIMER AND CACHE_LINE_SIZE are
> > arriving together in one call to xen_pcibk_config_write.
> Tweaks to make individual devices work are dubious. Any
> explanation should outline why current behavior is _generally_
> wrong. In particular with the original issue being with the
> Latency Timer field, and with that field now being allowed to
> be written by its entry in header_common[], ...
>
> > --- a/drivers/xen/xen-pciback/conf_space.c
> > +++ b/drivers/xen/xen-pciback/conf_space.c
> > @@ -230,8 +230,7 @@ int xen_pcibk_config_write(struct pci_dev *dev, int
> > offset, int size, u32 value)
> >                 field_start = OFFSET(cfg_entry);
> >                 field_end = OFFSET(cfg_entry) + field->size;
> >   
> > -		if ((req_start >= field_start && req_start < field_end)
> > -                   || (req_end > field_start && req_end <= field_end)) {
> > +                if (req_end >= field_start || field_end >= req_start) {
> >                         tmp_val = 0;
> ... any change to the logic here which results in writes to the field
> getting issued would seem wrong without even looking at the
> particular nature of the field.
>
> As to the actual change - the two _end variables hold values
> pointing _past_ the region of interest, so the use of >= seems
> wrong here (ought to be >). And in the end we're talking of a
> classical overlap check here, which indeed can be simplified (to
> just two comparisons), but such simplification mustn't result in a
> change of behavior. (Such a simplification would end up being
>
>                 if (req_end > field_start && field_end > req_start) {
>
> afaict - note the || instead of && used in your change.)
Will setting header_common[]'s PCI_CACHE_LINE_SIZE size field to 2 (and 
adding a proper comment) solve this problem?
-boris


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