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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.04.12 at 17:50, Lin Ming <mlin@xxxxxxxxxxxxx> 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.

I understand all of the above.

> 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.

Meaning you need to craft one if you need one, the more that
struct io_apic_ops already exists. The traditional kernel was able
to do so quite fine, and hence didn't need any hypercall.

Jan


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