From mboxrd@z Thu Jan  1 00:00:00 1970
From: Lin Ming <mlin@ss.pku.edu.cn>
Subject: Re: [RFC PATCH] x86: Add a new physdev_op
 PHYSDEVOP_nr_irqs_gsi
Date: Tue, 10 Apr 2012 23:50:08 +0800
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To: Jan Beulich <JBeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Xiantao Zhang <xiantao.zhang@intel.com>, xen-devel <xen-devel@lists.xen.org>
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On Tue, 2012-04-10 at 16:33 +0100, Jan Beulich wrote:
> >>> On 10.04.12 at 17:13, Lin Ming <mlin@ss.pku.edu.cn> 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.

Thanks,
Lin Ming

> 
> Jan