Re: [Xen-devel] [PATCH v6 6/8] AMD/IOMMU: tidy struct ivrs_mappings

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 24 September 2019 10:13
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: SuraveeSuthikulpanit <suravee.suthikulpanit@xxxxxxx>; Andrew Cooper 
> <Andrew.Cooper3@xxxxxxxxxx>;
> xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH v6 6/8] AMD/IOMMU: tidy struct ivrs_mappings
> 
> On 24.09.2019 11:08, Paul Durrant wrote:
> >> From: Jan Beulich <jbeulich@xxxxxxxx>
> >> Sent: 24 September 2019 10:02
> >>
> >> On 23.09.2019 18:25, Paul Durrant wrote:
> >>>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of 
> >>>> Jan Beulich
> >>>> Sent: 19 September 2019 14:24
> >>>>
> >>>> --- a/xen/include/asm-x86/amd-iommu.h
> >>>> +++ b/xen/include/asm-x86/amd-iommu.h
> >>>> @@ -106,12 +106,16 @@ struct amd_iommu {
> >>>>  };
> >>>>
> >>>>  struct ivrs_mappings {
> >>>> -    u16 dte_requestor_id;
> >>>> -    u8 dte_allow_exclusion;
> >>>> -    u8 unity_map_enable;
> >>>> -    u8 write_permission;
> >>>> -    u8 read_permission;
> >>>> +    uint16_t dte_requestor_id;
> >>>>      bool valid;
> >>>> +    bool dte_allow_exclusion;
> >>>> +    bool unity_map_enable;
> >>>> +    bool write_permission;
> >>>> +    bool read_permission;
> >>>
> >>> Could you shrink this even more by using a bit-field instead of this 
> >>> sequence of bools?
> >>
> >> Indeed I had been considering this. Besides the fact that making
> >> such a move simply didn't look to fit other things here very well
> >> when introducing the "valid" flag in an earlier path, and then
> >> also not here, do you realize though that this wouldn't shrink
> >> the structure's size right now (i.e. it would only be potentially
> >> reducing future growth)?
> >
> > Yes, I'd failed to note the 'unsigned long' afterwards, but...
> >
> >> This was my main argument against going
> >> this further step; let me know what you think.
> >>
> >
> > I still think a pre-emptive squash into a uint8_t bit-field followed
> > by the device_flags field would give the struct a nice 32-bit hole
> > for potential future use.
> 
> Okay, will do then. I take it your R-b can remain in place with this
> extra change.

Absolutely. Thanks,

  Paul

> 
> Jan
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