Re: [Qemu-devel] [PATCH] qemu: msi irq allocation api

[Michael S. Tsirkin]

On Thu, May 21, 2009 at 02:31:26PM +0100, Paul Brook wrote:
> On Thursday 21 May 2009, Paul Brook wrote:
> > > > MSI provides multiple edge triggered interrupts, whereas traditional
> > > > mode provides a single level triggered interrupt. My guess is most
> > > > devices will want to treat these differently anyway.
> > >
> > > So, is qemu_send_msi better than qemu_set_irq.
> >
> > Neither. pci_send_msi, which is a trivial wrapper around stl_phys.
> 
> To clarify, you seem to be trying to fuse two largely separate features 
> together.
> 
> MSI is a standard PCI device capability[1] that involves the device 
> performing 
> a 32-bit memory write when something interesting occurs. These writes may or 
> may not be directed at a APIC.
> 
> The x86 APIC has a memory mapped interface that allows generation of CPU 
> interrupts in response response to memory writes. These may or may not come 
> from an MSI capable PCI device.
> 
> Paul
> 
> [1] Note a *device* capability, not a bus capability.

Paul, so I went over specs, and what you say about APIC here does not
seem to be what Intel actually implemented.  Specifically, Intel
implemented *MSI support in APIC*. This lets PCI devices, but not the CPU,
signal interrupts by memory writes.

For example, after reset, when CPU writes to address 0xfee00000 this
is an access to a reserved register in APIC, but when PCI device
does write to 0xfee00000, this triggers an interrupt to destination 0.

See section 9.12 in Intel® 64 and IA-32 Architectures Software
Developer’s Manual Volume 3A: System Programming Guide, Part 1
http://www.intel.com/Assets/PDF/manual/253668.pdf

So it seems that what we need to do in pci is:

if (!msi_ops || msi_ops->send_msi(address, data))
        stl_phy(address, data);

where send_msi is wired to apic_send_msi and
where apic_send_msi returns an error for an address
outside of the MSI range 0xfee00000 - 0xfeefffff

Makes sense?

-- 
MST