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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 9 Nov 2009, Qing He wrote:
> 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.
> 

If "val always equals to read_val", why do we need to call
pci_write_block at all?


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