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Re: [PATCH RFC] vPCI: account for hidden devices in modify_bars()

On 31.08.21 11:35, Jan Beulich wrote:
On 31.08.2021 10:14, Oleksandr Andrushchenko wrote:
On 31.08.21 11:05, Jan Beulich wrote:
On 31.08.2021 09:56, Oleksandr Andrushchenko wrote:
On 31.08.21 10:47, Jan Beulich wrote:
On 31.08.2021 09:06, Oleksandr Andrushchenko wrote:
On 31.08.21 09:51, Jan Beulich wrote:
On 31.08.2021 07:35, Oleksandr Andrushchenko wrote:
On 30.08.21 16:04, Jan Beulich wrote:
@@ -265,7 +266,8 @@ static int modify_bars(const struct pci_
           * Check for overlaps with other BARs. Note that only BARs that are
           * currently mapped (enabled) are checked for overlaps.
-    for_each_pdev ( pdev->domain, tmp )
+for ( d = pdev->domain; ; d = dom_xen ) {//todo
I am not quite sure this will be correct for the cases where pdev->domain != 
e.g. in the series for PCI passthrough for Arm this can be any guest. For such 
we'll force running the loop for dom_xen which I am not sure is desirable.
It is surely not desirable, but it also doesn't happen - see the
is_hardware_domain() check further down (keeping context below).
Another question is why such a hidden device has its pdev->domain not set 
so we need to work this around?
Please see _setup_hwdom_pci_devices() and commit e46ea4d44dc0
("PCI: don't allow guest assignment of devices used by Xen")
introducing that temporary override. To permit limited
visibility to Dom0, these devices still need setting up in the
IOMMU for Dom0. Consequently BAR overlap detection also needs
to take these into account (i.e. the goal here is not just to
prevent triggering the ASSERT() in question).
So, why don't we set pdev->domain = dom_xen for such devices and call
modify_bars or something from pci_hide_device for instance (I didn't get too
much into implementation details though)? If pci_hide_device already handles
such exceptions, so it should also take care of the correct BAR overlaps etc.
How would it? It runs long before Dom0 gets created, let alone when
Dom0 may make adjustments to the BAR arrangement.
So, why don't we call "yet another hide function" while creating Dom0 for that
exactly reason, e.g. BAR overlap handling? E.g. make it 2-stage hide for special
devices such as console etc.
This might be an option, but is imo going to result not only in more
code churn, but also in redundant code. After all what modify_bars()
needs is the union of BARs from Dom0's and DomXEN's devices.
To me DomXEN here is yet another workaround as strictly speaking
vpci code didn't need and doesn't(?) need it at the moment. Yes, at least on 
So, I do understand why you want it there, but this then does need a very
good description of what is happening and why...

The temporary overriding of pdev->domain is because other IOMMU code
takes the domain to act upon from that field.
So, you mean pdev->domain in that case is pointing to what?
Did you look at the function I've pointed you at? DomXEN there gets
temporarily overridden to Dom0.
This looks like yet another workaround to me which is not cute.
So, the overall solution is spread over multiple subsystems, each
introducing something which is hard to follow
If you have any better suggestions, I'm all ears. Or feel free to send

Unfortunately I don't have any. But, could you please at least document a bit

more what is happening here with DomXEN: either in the commit message or

in the code itself, so it is easier to understand why vpci has this code at 

Thank you,





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