Re: [Xen-devel] [RFC PATCH] x86: Add a new physdev_op PHYSDEVOP_nr_irqs_gsi

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 11 Apr 2012, Zhang, Xiantao wrote:
> > -----Original Message-----
> > From: Stefano Stabellini [mailto:stefano.stabellini@xxxxxxxxxxxxx]
> > Sent: Wednesday, April 11, 2012 12:21 AM
> > To: Lin Ming
> > Cc: Jan Beulich; Konrad Rzeszutek Wilk; Zhang, Xiantao; xen-devel
> > Subject: Re: [Xen-devel] [RFC PATCH] x86: Add a new physdev_op
> > PHYSDEVOP_nr_irqs_gsi
> > 
> > On Tue, 10 Apr 2012, Lin Ming wrote:
> > > On Tue, 2012-04-10 at 16:33 +0100, Jan Beulich wrote:
> > > > >>> On 10.04.12 at 17:13, Lin Ming <mlin@xxxxxxxxxxxxx> wrote:
> > > > > This new physdev_op is added for Linux guest kernel to get the
> > > > > correct nr_irqs_gsi value.
> > > >
> > > > I'm not convinced this is really needed - the kernel can work out
> > > > the right number without any hypercall afaict.
> > >
> > > In Linux kernel:
> > >
> > > mp_register_ioapic(...):
> > >
> > >         entries = io_apic_get_redir_entries(idx);
> > >         gsi_cfg = mp_ioapic_gsi_routing(idx);
> > >         gsi_cfg->gsi_base = gsi_base;
> > >         gsi_cfg->gsi_end = gsi_base + entries - 1;
> > >
> > >         /*
> > >          * The number of IO-APIC IRQ registers (== #pins):
> > >          */
> > >         ioapics[idx].nr_registers = entries;
> > >
> > >         if (gsi_cfg->gsi_end >= gsi_top)
> > >                 gsi_top = gsi_cfg->gsi_end + 1;
> > >
> > > io_apic_get_redir_entries calls io_apic_read(), which returns wrong
> > > value(0xFFFFFFFF) on Xen Dom0 kernel.
> > >
> > > How can we get the correct gsi_top value, which is used to set
> > > nr_irqs_gsi, without hypercall?
> > >
> > > The problem here is we don't have a Xen version of io_apic_read in
> > > Linux kernel.
> > 
> > Actually we do: http://marc.info/?l=linux-kernel&m=133295662314519
> 
> This is not enough, seems fixed value are returned for each read ?

Yes, it looks like we are always returning 0x00170020, that means 24
entries that I guess is correct for the vast majority of io_apics.

We could limit ourselves to adding a comment saying "we assume the
io_apic has 24 redir entries".

Or we could use PHYSDEVOP_apic_read to do an actual io_apic read for
reg 0x1.

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