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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 25 Jun 2015, Jan Beulich wrote:
> >>> 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.

Right. The host cache idea is only good at init time I think.

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