Re: [Xen-devel] [v3 06/15] vmx: Extend struct pi_desc to support VT-d Posted-Interrupts

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.07.15 at 10:26, <feng.wu@xxxxxxxxx> wrote:

> 
>> -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Wednesday, July 15, 2015 4:20 PM
>> To: Wu, Feng
>> Cc: andrew.cooper3@xxxxxxxxxx; george.dunlap@xxxxxxxxxxxxx; Tian, Kevin;
>> Zhang, Yang Z; xen-devel@xxxxxxxxxxxxx; keir@xxxxxxx 
>> Subject: RE: [v3 06/15] vmx: Extend struct pi_desc to support VT-d
>> Posted-Interrupts
>> 
>> >>> On 15.07.15 at 04:40, <feng.wu@xxxxxxxxx> wrote:
>> 
>> >
>> >> -----Original Message-----
>> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> Sent: Friday, July 10, 2015 9:08 PM
>> >> To: Wu, Feng
>> >> Cc: andrew.cooper3@xxxxxxxxxx; george.dunlap@xxxxxxxxxxxxx; Tian, Kevin;
>> >> Zhang, Yang Z; xen-devel@xxxxxxxxxxxxx; keir@xxxxxxx 
>> >> Subject: Re: [v3 06/15] vmx: Extend struct pi_desc to support VT-d
>> >> Posted-Interrupts
>> >>
>> >> >>> On 24.06.15 at 07:18, <feng.wu@xxxxxxxxx> wrote:
>> >> > @@ -81,8 +81,19 @@ struct vmx_domain {
>> >> >
>> >> >  struct pi_desc {
>> >> >      DECLARE_BITMAP(pir, NR_VECTORS);
>> >> > -    u32 control;
>> >> > -    u32 rsvd[7];
>> >> > +    union {
>> >> > +        struct
>> >> > +        {
>> >> > +        u16 on     : 1,  /* bit 256 - Outstanding Notification */
>> >> > +            sn     : 1,  /* bit 257 - Suppress Notification */
>> >> > +            rsvd_1 : 14; /* bit 271:258 - Reserved */
>> >> > +        u8  nv;          /* bit 279:272 - Notification Vector */
>> >> > +        u8  rsvd_2;      /* bit 287:280 - Reserved */
>> >> > +        u32 ndst;        /* bit 319:288 - Notification Destination */
>> >> > +        };
>> >> > +        u64 control;
>> >> > +    };
>> >>
>> >> So current code, afaics, uses e.g. test_and_set_bit() to set ON.
>> >> By also declaring this as a bitfield you're opening the structure for
>> >> non-atomic accesses. If that's correct, why is other code not
>> >> being changed to _only_ use the bitfield mechanism (likely also
>> >> eliminating the need for it being a union with the now 64-bit
>> >> "control"? If atomic accesses are required, then I'd strongly
>> >> suggest against making this a bit field.
>> >>
>> >> And in no event can I see why "ndst" needs to be union-ized
>> >> with "control" if it doesn't need to be updated atomically with
>> >> e.g. "nv".
>> >>
>> >
>> > When the vCPU is to be blocked, we need to atomically update
>> > the "nv" and "ndst", then the wakeup notification event can be
>> > delivered to the right destination.
>> 
>> Okay. Your reply made me go through the patches again to check
>> where updates to nv/ndst happen - what's the reason they aren't
>> being updated as a pair in patch 14's RUNSTATE_running handling
>> (or in the replacement draft's vmx_ctxt_switch_to() adjustment)?
> 
> It is because, we can only enter running state from runnable, in which,
> the NV field has been already changed back to ' posted_intr_vector ',
> we don't need to do it here again.

Without sitting in the runstate update path anymore, I can't see how
you would get to see all transitions to runnable.

Jan


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