Re: [Xen-devel] [PATCH QEMU-XEN] xen/pt: Start with emulated PCI_COMMAND set to zero.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.06.15 at 17:59, <konrad.wilk@xxxxxxxxxx> wrote:
> While digging in this I realized that some shortcuts and assumptions
> had been taken (I think I am restating what you two have already
> realized).
> 
> 1) The dev.config is (by Xen code) used as the cache of the
>    host configuration devices (which is OK at init right now).
> 
>    However to sync up the ->data with dev.config (and apply emu_mask)
>    means that it cannot be used as such (the semantics of that have 
> changed). 
> 
> 2). The dev.config is (by the generic code) used as a view of what
>    the guest should see (cache of guest values).
> 
> 
> To make this work, the plan would be:
> 
>  1). Remove all the dev.config accesses to check host values:
>       @@ -728,7 +729,7 @@ static int xen_pt_initfn(PCIDevice *d)
>            }
>        
>            /* Bind interrupt */
>       -    if (!s->dev.config[PCI_INTERRUPT_PIN]) {
>       +    if (xen_host_pci_get_byte(PCI_INTERRUPT_PIN)) {
>                XEN_PT_LOG(d, "no pin interrupt\n");
>                goto out;
>            }
>      And replace them with calls to get the actual host value.
> 
>  2). Replace all the pci_get_[byte,word,long] (which are wrappers
>      around dev.config) in the init routines (see get_capability_version and
>      xen_pt_linkctrl_reg_init) with calls to xen_host_pci_[byte,word,long].
> 
>      In short - replace the calls to get the actual host values.
> 
> That would untangle a lot of this and make it a bit saner (I hope).

Except that I don't think changing the init-time uses would really be
necessary.

> And after that I can:
> 
>  3). Rip out ->data and use pci_set_[byte,word,long] or 
> pci_get_[byte,word,long]
>      to get one unified view of what the guest is suppose to have.
> 
>  4). Tackle bugs that will creep up because of this. I am not sure what
>      they are, but surely will hit some. I expect that some of these
>      patches will add debug/more logging to help me tackle this.
> 
>  5). Reinstate an host cache (if needed) for configuration access to lessen
>      the amount of reads we do. 'host_cache_config' perhaps?

Not sure it's a good idea to cache registers that can change behind
our back.

Jan


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