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

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

Hi Jan, Roger,

Jan Beulich <jbeulich@suse.com> 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. 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?

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

-- 
WBR, Volodymyr