On Thu, 2 Mar 2017, Andi Kleen wrote: > From: Andi Kleen <ak@xxxxxxxxxxxxxxx> > > 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
