Messages in this thread | | | Date | Tue, 14 Mar 2017 14:06:13 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH 1/4] pci: Allow lockless access path to PCI mmconfig |
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On Thu, 2 Mar 2017, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com> > > The Intel uncore driver can do a lot of PCI config accesses to read > performance counters. I had a situation on a 4S system where it > was spending 40+% of CPU time grabbing the pci_cfg_lock due to that. > > For 64bit x86 with MMCONFIG there isn't really any reason to take > a lock. The access is directly mapped to an underlying MMIO area, > which can fully operate lockless. > > Add a new flag that allows the PCI mid layer to skip the lock > and set it for the 64bit mmconfig code. > > There's a small risk that someone relies on this lock for synchronization, > but I think that's unlikely because there isn't really any useful > synchronization at this individual operation level. Any useful > synchronization would likely need to protect at least a > read-modify-write or similar. So I made it unconditional without opt-in.
This part of the changelog is just crap.
The reason why pci_lock exists and is taken for each single read/write config is that some ops implementations, e.g. the generic ones, must protect at this granularity level because
ops->map_bus() read/writeX()
needs to be 'atomic'.
MMCONFIG obviously does not require this at all because it's a simple byte/word/dword read/write which is serialized by itself. So it's obvious that the serialization with pci_lock is pointless in this case.
It's not that hard to figure it out and write up a proper changelog instead of handwaving about risk and whatever.
Thanks,
tglx
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