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

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:


        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

Lin Ming

> Jan

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