RE: [PATCH] PCI: hv: Do not set PCI_COMMAND_MEMORY to reduce VM boot time

[Jake Oshins]

> -----Original Message-----
> From: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Sent: Tuesday, April 26, 2022 11:32 AM
> To: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>
> Cc: Jake Oshins <jakeo@xxxxxxxxxxxxx>; Bjorn Helgaas <helgaas@xxxxxxxxxx>;
> bhelgaas@xxxxxxxxxx; Alex Williamson <alex.williamson@xxxxxxxxxx>;
> wei.liu@xxxxxxxxxx; KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang
> <haiyangz@xxxxxxxxxxxxx>; Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>;
> linux-hyperv@xxxxxxxxxxxxxxx; linux-pci@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; Michael Kelley (LINUX) <mikelley@xxxxxxxxxxxxx>;
> robh@xxxxxxxxxx; kw@xxxxxxxxx; kvm@xxxxxxxxxxxxxxx
> Subject: RE: [PATCH] PCI: hv: Do not set PCI_COMMAND_MEMORY to reduce
> VM boot time
> 
> > From: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>
> > 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?
> 

First, let me say that I really don't know whether this is an issue.  I know it's an issue with other operating system kernels.  I'm curious whether the Linux kernel / Linux PCI driver would behave in a way that has an issue here.

The VM has a window of address space into which it chooses to put PCI device's BARs.  The guest OS will generally pick the value that is within the BAR, by default, but it can theoretically place the device in any free address space.  The subset of the VM's memory address space which can be populated by devices' BARs is finite, and generally not particularly large.

Imagine a VM that is configured with 25 NVMe controllers, each of which requires 64KiB of address space.  (This is just an example.)  At first boot, all of these NVMe controllers are packed into address space, one after the other.

While that VM is running, one of the 25 NVMe controllers fails and is replaced with an NVMe controller from a separate manufacturer, but this one requires 128KiB of memory, for some reason.  Perhaps it implements the "controller buffer" feature of NVMe.  It doesn't fit in the hole that was vacated by the failed NVMe controller, so it needs to be placed somewhere else in address space.  This process continues over months, with several more failures and replacements.  Eventually, the address space is very fragmented.

At some point, there is an attempt to place an NVMe controller into the VM but there is no contiguous block of address space free which would allow that NVMe controller to operate.  There is, however, enough total address space if the other, currently functioning, NVMe controllers are moved from the address space that they are using to other ranges, consolidating their usage and reducing fragmentation.  Let's call this a rebalancing of memory resources.

When the NVMe controllers are moved, a new value is written into their BAR.  In general, the PCI spec would require that you clear the memory enable bit in the command register (PCI_COMMAND_MEMORY) during this move operation, both so that there's never a moment when two devices are occupying the same address space and because writing a 64-bit BAR atomically isn't possible.  This is the reason that I originally wrote the code in this driver to unmap the device from the VM's address space when the memory enable bit is cleared.

What I don't know is whether this sequence of operations can ever happen in Linux, or perhaps in a VM running Linux.  Will it rebalance resources in order to consolidate address space?  If it will, will this involve clearing the memory enable bit to ensure that two devices never overlap?

Thanks,
Jake Oshins