> From: Bjorn Helgaas <helgaas@xxxxxxxxxx> > Sent: Wednesday, June 13, 2018 15:15 > > ... > > It looks Lorenzo's pci.git tree has not been updated for 3+ weeks. > > I guess Lorenzo may be on vacation. > > > > @Bjorn, can this patch go through your tree? > > Should I resubmit it? > > No need to resubmit it, Lorenzo has been out for a bit, but I'm sure > he'll pick this up as he catches up. OK, I see. Thanks! > You might, however, fix the commit log: > > This is not an issue because hv_pci_onchannelcallback() is not slow, > and it not a hot path. > > This has at least one typo (I think you mean "and *is* not a hot > path"). Sorry -- yes, it's a typo. I hope Lorenzo can help to fix this, or I can resubmit it if Lorenzo or you want me to do it. > I also don't understand the sentence as a whole because the > hv_pci_onchannelcallback() comment says it's called whenever the host > sends a packet to this channel, and that *does* sound like a hot path. Sorry for not making it clear. The host only sends a packet into the channel of the guest when there is a change of device configuration (i.e. hot add or remove a device), or the host is responding to the guest's request. The change of device configuration is only triggered on-demand by the administrator on the host, and the guest's requests are one-off when the device is probed. So IMO the callback is not a hot path. > I also don't understand the "hv_pci_onchannelcallback() is not slow" > part. In other words, you're saying hv_pci_onchannelcallback() is > fast and it's not a hot path. And apparently this has something to do > with the difference between local_bh_disable() and local_irq_save()? > > Bjorn Actually in my original internal version of the patch, I did use local_irq_save/restore(). hv_pci_onchannelcallback() itself runs fast, but here since it's in a loop (i.e. the while (!try_wait_for_completion(&comp.comp_pkt.host_event) loop), IIRC I was asked if I really need local_irq_save/restore(), and I answered "not really", so later I switched to local_bh_disable()/enable(). However, recently I found that if we enable CONFIG_PROVE_LOCKING=y, the local_bh_enable() can trigger a warning because the function hv_compose_msi_msg() can be called with local IRQs disabled (BTW, hv_compose_msi_msg() can also be called with local IRQS enabled in another code path): IRQs not enabled as expected WARNING: CPU: 0 PID: 408 at kernel/softirq.c:162 __local_bh_enable_ip Despite the warning, the code itself can still work correctly, but IMO we'd better switch back to local_irq_save/restore(), and hence I made the patch. I hope the explanation sounds reasonable. :-) Thanks, -- Dexuan
