linux-kernel - RE: [PATCH] PCI: hv: Do not set PCI_COMMAND

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Date:   Tue, 26 Apr 2022 18:31:56 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
CC:     Jake Oshins <jakeo@...rosoft.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "robh@...nel.org" <robh@...nel.org>, "kw@...ux.com" <kw@...ux.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [PATCH] PCI: hv: Do not set PCI_COMMAND_MEMORY to reduce VM boot
 time

> From: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> Sent: Tuesday, April 26, 2022 9:45 AM
> > ...
> > Sorry I don't quite follow. pci-hyperv allocates MMIO for the bridge
> > window in hv_pci_allocate_bridge_windows() and registers the MMIO
> > ranges to the core PCI driver via pci_add_resource(), and later the
> > core PCI driver probes the bus/device(s), validates the BAR sizes and
> > the pre-initialized BAR values, and uses the BAR configuration. IMO
> > the whole process doesn't require the bit PCI_COMMAND_MEMORY to be
> > pre-set, and there should be no issue to delay setting the bit to a
> > PCI device device's .probe() -> pci_enable_device().
> 
> IIUC you want to bootstrap devices with PCI_COMMAND_MEMORY clear
> (otherwise PCI core would toggle it on and off for eg BAR sizing).
> 
> Is that correct ?

Yes, that's the exact purpose of this patch.

Do you see any potential architectural issue with the patch? 
>From my reading of the core PCI code, it looks like this should be safe.

Jake has some concerns that I don't quite follow. 
@Jake, could you please explain the concerns with more details?

> If I read PCI core correctly PCI_COMMAND_MEMORY is obviously cleared
> only if it is set in the first place and that's what your patch is
> changing, namely you boostrap your devices with PCI_COMMAND_MEMORY
> clear so that PCI core does not touch it.

Yes, this is what exactly the patch is doing.

Thanks,
-- Dexuan