Re: [RFC PATCH] pci: introduce per-domain PCI rwlock

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.07.2023 13:09, Volodymyr Babchuk wrote:
> Jan Beulich <jbeulich@xxxxxxxx> writes:
> 
>> Up front remark: I'm sorry for some possibly unhelpful replies below. I,
>> for one, am of the opinion that some of the issues you ask about are to
>> be looked into by whoever wants / needs to rework the locking model.
>> After all this (likely) is why nobody has dared to make an attempt before
>> the need became apparent.
> 
> I have no great need desire or need to rework the locking model. I was
> perfectly fine with much narrower vpci_lock. As you remember, it is
> Roger who suggested to extend this lock to the include the whole PCI
> device.
> 
> I already tried something like this as part of the another patch series:
> "[RFC,01/10] xen: pci: add per-domain pci list lock" [1], with the same
> result: it was considered very hard to do this properly, so I dropped
> that effort. I am not so familiar with x86-specific code as a whole and
> IOMMU drivers in particular to be 100% sure that I am doing correct
> changes. Without support from x86 guys I can't do proper patches and
> looks like x86 guys are not interested in this.

That's not the case, no. The problem is time: I don't have the time to
take on this effort myself. I'm willing to help where necessary, within
reasonable bounds. But I can't realistically do large parts of the
analysis that is inevitably needed. (I'm also a little sick of doing
code audits for other, unrelated reasons.) Hence that earlier "up front"
remark.

> So, this is dead end.
> 
> Roger, in [2] I proposed another approach to fix ABBA in modify_bars():
> store copy of BARs in the domain structure. Taking into account that my
> effort to introduce d->pci_lock basically failed (again), I am proposing
> to return back to d->vpci_lock + BARs shadow copy in the domain
> struct. What do you think? And you, Jan?

I support Roger's earlier request, and I think that doing what you
propose would move us further away from where we want to arrive at some
point.

I'm sorry that this is all pretty unpleasant.

Jan

> [1] 
> https://lore.kernel.org/all/20220831141040.13231-2-volodymyr_babchuk@xxxxxxxx/
> [2] https://lore.kernel.org/all/87ilbfnqmo.fsf@xxxxxxxx/
>