Re: [Qemu-devel] [RFC PATCH v2 07/21] ppc/xive: add MMIO handlers for the XIVE interrupt sources

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2017-09-20 at 15:05 +0200, Cédric Le Goater wrote:
> > > +/*
> > > + * XIVE Interrupt Source MMIOs
> > > + */
> > > +static uint64_t spapr_xive_esb_read(void *opaque, hwaddr addr, unsigned size)
> > > +{
> > > +    sPAPRXive *xive = SPAPR_XIVE(opaque);
> > > +    uint32_t offset = addr & 0xF00;
> > > +    uint32_t srcno = addr >> xive->esb_shift;
> > > +    XiveIVE *ive;
> > > +    uint64_t ret = -1;
> > > +
> > > +    ive = spapr_xive_get_ive(xive, srcno);
> > > +    if (!ive || !(ive->w & IVE_VALID))  {
> > > +        qemu_log_mask(LOG_GUEST_ERROR, "XIVE: invalid LISN %d\n", srcno);
> > > +        goto out;
> > 
> > Since there's a whole (4k) page for each source, I wonder if we should
> > actually map each one as a separate MMIO region to allow us to tweak
> > the mappings more flexibly
> 
> yes we could have a subregion for each source. In that case, 
> we should also handle IVE_VALID properly. That will require 
> a specific XIVE allocator which was difficult to do while
> keeping the compatibility with XICS for migration and CAS.

That will be a serious bloat with lots of interrupts. We also cannot
possibly have a KVM mm region per interrupt or even a vma.

I'm thinking of some kind of /dev/xive (or some other KVM or irqfd
orignated fd) that allows you to mmap a single big region whose content
is demand-faulted and invalidated by the kernel to map the various
interrupts.

So that it looks like a single VMA (and KVM memory block).

Ben.

> C.
> 
>