Re: [Xen-devel] [PATCH] ioemu: passthough: add no_wb option for pci conf write

[Qing He <qing.he@xxxxxxxxx>]

On Sat, 2009-11-07 at 02:16 +0800, Ian Jackson wrote:
> But I do have a question about it.  I hope you'll forgive my ignorance
> about MSIs (I haven't read the reference manuals).
> 
> I don't think I fully understand the problem this is trying to fix.

I know that patch note is not clear enough :-)
> 
> There are two ways of updating the MSI address and data registers ?
> Are they available via the space directly mapped into the guest as
> well as via config space then ?

The data and address registers is the sole way to update MSI vector and
affinity (at least when not using intremap), but the problem here is
that QEmu overwrite the hypervisor changes using stale data.

As we know, guest MSI is virtual, this means guest MSI address and data
are all emulated, and guest vector has nothing to do with real vector.
QEmu needs to map and bind MSI through Xen. via the following two calls:

        xc_physdev_map_pirq_msi
        xc_domain_bind_pt_irq

The physical content of MSI data/address is then decided and written by Xen.
xc_physdev_map_pirq_msi is also used to update guest MSI, including vector
and affinity.

Now come to the pt_pci_write_config logic:

        pci_read_block(&read_val);
        reg->u.dw.write(read_val, &val);   // the handler
        pci_write_block(val);

Since MSI data/address is fully emulated, val always equals to read_val,
i.e. write what is read back to the register. This would be OK for most of
the time, however, when the guest changes MSI affinity, something happens
between read and write. the handler calls xc_physdev_map_pirq_msi to update
the MSI, hypervisor changes the affinity and write a new vector/affinity
to the real registers. When the handler returns, pci_write_block(val)
overwrites the real registers, all the HV changes are lost, making the
MSI fail.

Thanks,
Qing

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