RE: [RFC PATCH v4 2/6] vfio: Introduce vGPU display irq type

[Zhang, Tina]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Tuesday, July 23, 2019 9:19 AM
> To: Zhang, Tina <tina.zhang@xxxxxxxxx>
> Cc: Lu, Kechen <kechen.lu@xxxxxxxxx>; intel-gvt-dev@xxxxxxxxxxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; kraxel@xxxxxxxxxx;
> zhenyuw@xxxxxxxxxxxxxxx; Lv, Zhiyuan <zhiyuan.lv@xxxxxxxxx>; Wang, Zhi A
> <zhi.a.wang@xxxxxxxxx>; Tian, Kevin <kevin.tian@xxxxxxxxx>; Yuan, Hang
> <hang.yuan@xxxxxxxxx>
> Subject: Re: [RFC PATCH v4 2/6] vfio: Introduce vGPU display irq type
> 
> On Tue, 23 Jul 2019 01:08:19 +0000
> "Zhang, Tina" <tina.zhang@xxxxxxxxx> wrote:
> 
> > > -----Original Message-----
> > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> > > Sent: Tuesday, July 23, 2019 3:41 AM
> > > To: Lu, Kechen <kechen.lu@xxxxxxxxx>
> > > Cc: intel-gvt-dev@xxxxxxxxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-
> > > kernel@xxxxxxxxxxxxxxx; Zhang, Tina <tina.zhang@xxxxxxxxx>;
> > > kraxel@xxxxxxxxxx; zhenyuw@xxxxxxxxxxxxxxx; Lv, Zhiyuan
> > > <zhiyuan.lv@xxxxxxxxx>; Wang, Zhi A <zhi.a.wang@xxxxxxxxx>; Tian,
> > > Kevin <kevin.tian@xxxxxxxxx>; Yuan, Hang <hang.yuan@xxxxxxxxx>
> > > Subject: Re: [RFC PATCH v4 2/6] vfio: Introduce vGPU display irq
> > > type
> > >
> > > On Mon, 22 Jul 2019 05:28:35 +0000
> > > "Lu, Kechen" <kechen.lu@xxxxxxxxx> wrote:
> > >
> > > > Hi,
> > > >
> > > > > -----Original Message-----
> > > > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> > > > > Sent: Saturday, July 20, 2019 12:25 AM
> > > > > To: Lu, Kechen <kechen.lu@xxxxxxxxx>
> > > > > Cc: intel-gvt-dev@xxxxxxxxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> > > > > linux- kernel@xxxxxxxxxxxxxxx; Zhang, Tina
> > > > > <tina.zhang@xxxxxxxxx>; kraxel@xxxxxxxxxx;
> > > > > zhenyuw@xxxxxxxxxxxxxxx; Lv, Zhiyuan <zhiyuan.lv@xxxxxxxxx>;
> > > > > Wang, Zhi A <zhi.a.wang@xxxxxxxxx>; Tian, Kevin
> > > > > <kevin.tian@xxxxxxxxx>; Yuan, Hang <hang.yuan@xxxxxxxxx>
> > > > > Subject: Re: [RFC PATCH v4 2/6] vfio: Introduce vGPU display irq
> > > > > type
> > > > >
> > > > > On Thu, 18 Jul 2019 23:56:36 +0800 Kechen Lu
> > > > > <kechen.lu@xxxxxxxxx> wrote:
> > > > >
> > > > > > From: Tina Zhang <tina.zhang@xxxxxxxxx>
> > > > > >
> > > > > > Introduce vGPU specific irq type VFIO_IRQ_TYPE_GFX, and
> > > > > > VFIO_IRQ_SUBTYPE_GFX_DISPLAY_IRQ as the subtype for vGPU
> > > > > > display
> > > > > >
> > > > > > Signed-off-by: Tina Zhang <tina.zhang@xxxxxxxxx>
> > > > > > ---
> > > > > >  include/uapi/linux/vfio.h | 3 +++
> > > > > >  1 file changed, 3 insertions(+)
> > > > > >
> > > > > > diff --git a/include/uapi/linux/vfio.h
> > > > > > b/include/uapi/linux/vfio.h index be6adab4f759..df28b17a6e2e
> > > > > > 100644
> > > > > > --- a/include/uapi/linux/vfio.h
> > > > > > +++ b/include/uapi/linux/vfio.h
> > > > > > @@ -469,6 +469,9 @@ struct vfio_irq_info_cap_type {
> > > > > >  	__u32 subtype;  /* type specific */  };
> > > > > >
> > > > > > +#define VFIO_IRQ_TYPE_GFX				(1)
> > > > > > +#define VFIO_IRQ_SUBTYPE_GFX_DISPLAY_IRQ		(1)
> > > > > > +
> > > > >
> > > > > Please include a description defining exactly what this IRQ is
> > > > > intended to
> > > signal.
> > > > > For instance, if another vGPU vendor wanted to implement this in
> > > > > their driver and didn't have the QEMU code for reference to what
> > > > > it does with the IRQ, what would they need to know?  Thanks,
> > > > >
> > > > > Alex
> > > > >
> > > >
> > > > Yes, that makes more sense. I'll add the description for it at
> > > > next version
> > > patch.
> > > >
> > > > BTW, may I have one more question? In the current design ideas, we
> > > > partitioned the vGPU display eventfd counted 8-byte value into at
> > > > most
> > > > 8 events to deliver multiple display events, so we need different
> > > > increasement counter value to differentiate the events. As this is
> > > > the exposed thing the QEMU has to know, we plan adds a macro here
> > > > VFIO_IRQ_SUBTYPE_GFX_DISPLAY_EVENTFD_BASE_SHIFT to make sure
> > > the
> > > > partitions shift in 1 byte, does it make sense putting here?
> > > > Looking forward
> > > to your and Gerd's comments. Thanks!
> > >
> > > Couldn't you expose this as another capability within the IRQ_INFO
> > > return data?  If you were to define it as a macro, I assume that
> > > means it would be hard coded, in which case this probably becomes an
> > > Intel specific IRQ, rather than what appears to be framed as a
> > > generic graphics IRQ extension.  A new capability could instead
> > > allow the vendor to specify their own value, where we could define how
> userspace should interpret and make use of this value.
> > > Thanks,
> > Good suggestion. Currently, vfio_irq_info is used to save one irq
> > info. What we need here is to use it to save several events info.
> > Maybe we could figure out a general layout of this capability so that
> > it can be leveraged by others, not only for display irq/events.
> 
> You could also expose a device specific IRQ with count > 1 (ie. similar to
> MSI/X) and avoid munging the eventfd value, which is not something we do
> elsewhere, at least in vfio.  Thanks,
Actually, we had this implementation before. At that time, we got the suggestion that count > 1 means more than one eventfd which might be not necessary.
Anyway, we can consider the "count > 1" again if anyone is agree on this. Thanks

BR,
Tina

> 
> Alex