Re: [PATCH v5 08/11] xen/arm: Enable the existing x86 virtual PCI support for ARM.

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Jan,

> On 13 Oct 2021, at 07:10, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
> On 12.10.2021 23:37, Stefano Stabellini wrote:
>> On Tue, 12 Oct 2021, Jan Beulich wrote:
>>> On 11.10.2021 20:18, Stefano Stabellini wrote:
>>>> On Mon, 11 Oct 2021, Jan Beulich wrote:
>>>>> On 11.10.2021 15:34, Bertrand Marquis wrote:
>>>>>>> On 11 Oct 2021, at 14:09, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>>>>> On 11.10.2021 14:41, Bertrand Marquis wrote:
>>>>>>>> But digging deeper into this, I would have 2 questions:
>>>>>>>> 
>>>>>>>> - msi_cleanup was done there after a request from Stefano, but is not
>>>>>>>> done in case or iommu error, is there an issue to solve here ?
>>>>>>> 
>>>>>>> Maybe, but I'm not sure. This very much depends on what a domain
>>>>>>> could in principle do with a partly set-up device. Plus let's
>>>>>>> not forget that we're talking of only Dom0 here (for now at least,
>>>>>>> i.e. not considering the dom0less case).
>>>>>>> 
>>>>>>> But I'd also like to further defer to Stefano.
>>>>>> 
>>>>>> Ok, I must admit I do not really see at that stage why doing an MSI 
>>>>>> cleanup
>>>>>> could be needed so I will wait for Stefano to know if I need to keep 
>>>>>> this when
>>>>>> moving the block up (at the end it is theoretical right now as this is 
>>>>>> empty).
>>>> 
>>>> I know that MSIs are not supported yet on ARM (pci_cleanup_msi does
>>>> nothing). But I wanted to make sure that the pci_cleanup_msi() calls are
>>>> present anywhere necessary, especially on the error paths. So that once
>>>> we add MSI support, we don't need to search through the code to find all
>>>> the error paths missing a pci_cleanup_msi() call.
>>>> 
>>>> To answer your first question: you are right, we are also missing a
>>>> pci_cleanup_msi() call in the case of IOMMU error. So it might be better
>>>> to move the call to pci_cleanup_msi() under the "out" label so that we
>>>> can do it once for both cases.
>>>> 
>>>> To answer your second point about whether it is necessary at all: if
>>>> MSIs and MSI-Xs cannot be already setup at this point at all (not even
>>>> the enable bit), then we don't need any call to pci_cleanup_msi() in
>>>> pci_add_device.
>>> 
>>> Well, at the very least MSI can't be set up ahead of the traps getting
>>> put in place. Whether partial success of putting traps in place may
>>> allow a cunning guest to set up MSI may depend on further aspects.
>> 
>> Good point about MSIs not being setup before the traps. We should remove
>> the call to pci_cleanup_msi() in the error path then.
> 
> Your reply makes me fear you didn't pay enough attention to the "partial"
> in my earlier reply. The traps for the various registers can't all be set
> up atomically, so there may be a transient period where enough traps are
> already in place for a cunning guest to arrange for setup. Unless, as
> said, there are further setup steps needed before a guest could succeed
> in doing so.
> 
> But even if partial trap setup alone was sufficient, I think the cleaning
> up of MSI then might still better go on the error path there than on that
> of pci_add_device().

I think I should put the msi_cleanup in the exit path if pdev is not null but
we got a non null ret (in an else if ( pdev ) ).
This would cover all exit paths, especially as I will move the add_handler
before the iommu init.

Would that be ok for everyone ?

Cheers
Bertrand