Re: [PATCH 09/11] xen/arm: Setup MMIO range trap handlers for hardware domain

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 14 Sep 2021, Oleksandr Andrushchenko wrote:
> >> What you want to know if how many times register_mmio_handler() will be 
> >> called from domain_vpci_init().
> >>
> >> You introduced a function pci_host_iterate_bridges() that will walk over 
> >> the bridges and then call the callback vpci_setup_mmio_handler(). So you 
> >> could introduce a new callback that will return 1 if 
> >> bridge->ops->register_mmio_handler is not NULL or 0.
> >
> > Ok, clear. Something like:
> >
> >     if ( (rc = domain_vgic_register(d, &count)) != 0 )
> >         goto fail;
> >
> >     *find out how many bridges and update count*
> >
> >
> >     if ( (rc = domain_io_init(d, count + MAX_IO_HANDLER)) != 0 )
> >         goto fail;
> >
> I have the following code now:
> 
> int domain_vpci_get_num_mmio_handlers(struct domain *d)
> {
>      int count;

count is incremented but not initialized


>      if ( is_hardware_domain(d) )
>          /* For each PCI host bridge's configuration space. */
>          count += pci_host_get_num_bridges();
>      else
>          /*
>           * VPCI_MSIX_MEM_NUM handlers for MSI-X tables per each PCI device
>           * being passed through. Maximum number of supported devices
>           * is 32 as virtual bus topology emulates the devices as embedded
>           * endpoints.
>           * +1 for a single emulated host bridge's configuration space. */
>          count = VPCI_MSIX_MEM_NUM * 32 + 1;
>      return count;
> }
> 
> Please note that we cannot tell how many PCIe devices are going to be passed 
> through
> 
> So, worst case for DomU is going to be 65 to what we already have...
> 
> This sounds scary a bit as most probably we won't pass through 32 devices 
> most of the
> 
> time, but will make d->arch.vmmio.handlers almost 4 times bigger then it is 
> now.
> 
> This may have influence on the MMIO handlers performance...

I am OK with that given that it doesn't affect performance until you
actually start creating too many virtual devices for the DomU. In other
words, find_mmio_handler restricts the search to vmmio->num_entries, so
as long as most entries are allocated but unused, we should be fine.