Re: [Xen-devel] [v3 11/15] Update IRTE according to guest interrupt config changes

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Wu, Feng
> Sent: Wednesday, July 08, 2015 6:32 PM
> 
> 
> 
> > -----Original Message-----
> > From: Tian, Kevin
> > Sent: Wednesday, July 08, 2015 6:23 PM
> > To: Wu, Feng; xen-devel@xxxxxxxxxxxxx
> > Cc: keir@xxxxxxx; jbeulich@xxxxxxxx; andrew.cooper3@xxxxxxxxxx; Zhang,
> > Yang Z; george.dunlap@xxxxxxxxxxxxx
> > Subject: RE: [v3 11/15] Update IRTE according to guest interrupt config
> > changes
> >
> > > From: Wu, Feng
> > > Sent: Wednesday, June 24, 2015 1:18 PM
> > >
> > > When guest changes its interrupt configuration (such as, vector, etc.)
> > > for direct-assigned devices, we need to update the associated IRTE
> > > with the new guest vector, so external interrupts from the assigned
> > > devices can be injected to guests without VM-Exit.
> > >
> > > For lowest-priority interrupts, we use vector-hashing mechamisn to find
> > > the destination vCPU. This follows the hardware behavior, since modern
> > > Intel CPUs use vector hashing to handle the lowest-priority interrupt.
> > >
> > > For multicast/broadcast vCPU, we cannot handle it via interrupt posting,
> > > still use interrupt remapping.
> > >
> > > Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> > > ---
> > > v3:
> > > - Use bitmap to store the all the possible destination vCPUs of an
> > > interrupt, then trying to find the right destination from the bitmap
> > > - Typo and some small changes
> > >
> > >  xen/drivers/passthrough/io.c | 96
> > > +++++++++++++++++++++++++++++++++++++++++++-
> > >  1 file changed, 95 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/xen/drivers/passthrough/io.c b/xen/drivers/passthrough/io.c
> > > index 9b77334..18e24e1 100644
> > > --- a/xen/drivers/passthrough/io.c
> > > +++ b/xen/drivers/passthrough/io.c
> > > @@ -26,6 +26,7 @@
> > >  #include <asm/hvm/iommu.h>
> > >  #include <asm/hvm/support.h>
> > >  #include <xen/hvm/irq.h>
> > > +#include <asm/io_apic.h>
> > >
> > >  static DEFINE_PER_CPU(struct list_head, dpci_list);
> > >
> > > @@ -199,6 +200,78 @@ void free_hvm_irq_dpci(struct hvm_irq_dpci *dpci)
> > >      xfree(dpci);
> > >  }
> > >
> > > +/*
> > > + * The purpose of this routine is to find the right destination vCPU for
> > > + * an interrupt which will be delivered by VT-d posted-interrupt. There
> > > + * are several cases as below:
> >
> > If you aim to have this interface common to more usages, don't restrict to
> > VT-d posted-interrupt which should be just an example.
> 
> Yes, making this a common interface should be better.
> 
> >
> > > + *
> > > + * - For lowest-priority interrupts, we find the destination vCPU from 
> > > the
> > > + *   guest vector using vector-hashing mechanism and return true. This
> > follows
> > > + *   the hardware behavior, since modern Intel CPUs use vector hashing to
> > > + *   handle the lowest-priority interrupt.
> >
> > Does AMD use same hashing mechanism? Can this interface be reused by
> > other IOMMU type or it's an Intel specific implementation?
> 
> I am not sure how AMD handle lowest-priority. Intel hardware guys told me
> recent Intel hardware platform use this method to deliver lowest-priority
> interrupts. What do you mean by "other IOMMU type"?
> 

OS doesn't assume how vector hashing is done in hardware level. So it should
be fine to use Intel algorithm in this emulation path. However my point is just
about the comment " since modern Intel CPUs use vector hashing to handle 
the lowest-priority interrupt". It's not because Intel does so. It's the 
implementation option that you choose Intel algorithm here.

Thanks
Kevin

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