Linux-PCI Archive on lore.kernel.org
 help / color / Atom feed
From: Bjorn Helgaas <helgaas@kernel.org>
To: Sergey Miroshnichenko <s.miroshnichenko@yadro.com>
Cc: linux-pci@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux@yadro.com, Sam Bobroff <sbobroff@linux.ibm.com>,
	Rajat Jain <rajatja@google.com>, Lukas Wunner <lukas@wunner.de>,
	Oliver O'Halloran <oohall@gmail.com>,
	David Laight <David.Laight@ACULAB.COM>
Subject: Re: [PATCH v5 03/23] PCI: hotplug: Add a flag for the movable BARs feature
Date: Tue, 15 Oct 2019 17:14:49 -0500
Message-ID: <20191015221449.GA181069@google.com> (raw)
In-Reply-To: <92cf42bf-5044-329d-f1be-53b48801865f@yadro.com>

On Mon, Sep 30, 2019 at 03:59:25PM +0300, Sergey Miroshnichenko wrote:
> Hello Bjorn,
> 
> On 9/28/19 1:02 AM, Bjorn Helgaas wrote:
> > On Fri, Aug 16, 2019 at 07:50:41PM +0300, Sergey Miroshnichenko wrote:
> > > When hot-adding a device, the bridge may have windows not big enough (or
> > > fragmented too much) for newly requested BARs to fit in. And expanding
> > > these bridge windows may be impossible because blocked by "neighboring"
> > > BARs and bridge windows.
> > > 
> > > Still, it may be possible to allocate a memory region for new BARs with the
> > > following procedure:
> > > 
> > > 1) notify all the drivers which support movable BARs to pause and release
> > >     the BARs; the rest of the drivers are guaranteed that their devices will
> > >     not get BARs moved;
> > > 
> > > 2) release all the bridge windows except of root bridges;
> > > 
> > > 3) try to recalculate new bridge windows that will fit all the BAR types:
> > >     - fixed;
> > >     - immovable;
> > >     - movable;
> > >     - newly requested by hot-added devices;
> > > 
> > > 4) if the previous step fails, disable BARs for one of the hot-added
> > >     devices and retry from step 3;
> > > 
> > > 5) notify the drivers, so they remap BARs and resume.
> > 
> > You don't do the actual recalculation in *this* patch, but since you
> > mention the procedure here, are we confident that we never make things
> > worse?
> > 
> > It's possible that a hot-add will trigger this attempt to move things
> > around, and it's possible that we won't find space for the new device
> > even if we move things around.  But are we certain that every device
> > that worked *before* the hot-add will still work *afterwards*?
> > 
> > Much of the assignment was probably done by the BIOS using different
> > algorithms than Linux has, so I think there's some chance that the
> > BIOS did a better job and if we lose that BIOS assignment, we might
> > not be able to recreate it.
> 
> If a hardware has some special constraints on BAR assignment that the
> kernel is not aware of yet, the movable BARs may break things after a
> hotplug event. So the feature must be disabled there (manually) until
> the kernel get support for that special needs.

I'm not talking about special constraints on BAR assignment.  (I'm not
sure what those constraints would be -- AFAIK the constraints for a
spec-compliant device are all discoverable via the BAR size and type
(or the Enhanced Allocation capability)).

What I'm concerned about is the case where we boot with a working
assignment, we hot-add a device, we move things around to try to
accommodate the new device, and not only do we fail to find resources
for the new device, we also fail to find a working assignment for the
devices that were present at boot.  We've moved things around from
what BIOS did, and since we use a different algorithm than the BIOS,
there's no guarantee that we'll be able to find the assignment BIOS
did.

> > I'm not sure why the PCI_CLASS_DISPLAY_VGA special case is there; can
> > you add a comment about why that's needed?  Obviously we can't move
> > the 0xa0000 legacy frame buffer because I think devices are allowed to
> > claim that region even if no BAR describes it.  But I would think
> > *other* BARs of VGA devices could be movable.
> 
> Sure, I'll add a comment to the code.
> 
> The issue that we are avoiding by that is the "nomodeset" command line
> argument, which prevents a video driver from being bound, so the BARs
> are seems to be used, but can't be moved, otherwise machines just hang
> after hotplug events. That was the only special ugly case we've
> spotted during testing. I'll check if it will be enough just to work
> around the 0xa0000.

"nomodeset" is not really documented and is a funny way to say "don't
bind video drivers that know about it", but OK.  Thanks for checking
on the other BARs.

> > > +bool pci_movable_bars_enabled(void);
> > 
> > I would really like it if this were simply
> > 
> >    extern bool pci_no_movable_bars;
> > 
> > in drivers/pci/pci.h.  It would default to false since it's
> > uninitialized, and "pci=no_movable_bars" would set it to true.
> 
> I have a premonition of platforms that will not support the feature.
> Wouldn't be better to put this variable-flag to include/linux/pci.h ,
> so code in arch/* can set it, so they could work by default, without
> the command line argument?

In general I don't see why a platform wouldn't support this since
there really isn't anything platform-specific here.  But if a platform
does need to disable it, having arch code set this flag sounds
reasonable.  We shouldn't make it globally visible until we actually
need that, though.

> > We have similar "=off" and "=force" parameters for ASPM and other
> > things, and it makes the code really hard to analyze.

The "=off" and "=force" things are the biggest things I'd like to
avoid.

Bjorn

  reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-16 16:50 [PATCH v5 00/23] PCI: Allow BAR movement during hotplug Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 01/23] PCI: Fix race condition in pci_enable/disable_device() Sergey Miroshnichenko
2019-08-22 12:37   ` Marta Rybczynska
2019-09-27 21:59   ` Bjorn Helgaas
2019-09-30  8:53     ` Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 02/23] PCI: Enable bridge's I/O and MEM access for hotplugged devices Sergey Miroshnichenko
2019-09-27 22:01   ` Bjorn Helgaas
2019-08-16 16:50 ` [PATCH v5 03/23] PCI: hotplug: Add a flag for the movable BARs feature Sergey Miroshnichenko
2019-09-27 22:02   ` Bjorn Helgaas
2019-09-30  8:44     ` David Laight
2019-09-30 16:17       ` Sergey Miroshnichenko
2019-09-30 12:59     ` Sergey Miroshnichenko
2019-10-15 22:14       ` Bjorn Helgaas [this message]
2019-10-16 15:50         ` Sergey Miroshnichenko
2019-10-16 17:29           ` Bjorn Helgaas
2019-08-16 16:50 ` [PATCH v5 04/23] PCI: Define PCI-specific version of the release_child_resources() Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 05/23] PCI: hotplug: movable BARs: Fix reassigning the released bridge windows Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 06/23] PCI: hotplug: movable BARs: Recalculate all bridge windows during rescan Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 07/23] PCI: hotplug: movable BARs: Don't allow added devices to steal resources Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 08/23] PCI: Include fixed and immovable BARs into the bus size calculating Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 09/23] PCI: Prohibit assigning BARs and bridge windows to non-direct parents Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 10/23] PCI: hotplug: movable BARs: Try to assign unassigned resources only once Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 11/23] PCI: hotplug: movable BARs: Calculate immovable parts of bridge windows Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 12/23] PCI: hotplug: movable BARs: Compute limits for relocated " Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 13/23] PCI: Make sure bridge windows include their fixed BARs Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 14/23] PCI: Fix assigning the fixed prefetchable resources Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 15/23] PCI: hotplug: movable BARs: Assign fixed and immovable BARs before others Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 16/23] PCI: hotplug: movable BARs: Don't reserve IO/mem bus space Sergey Miroshnichenko
2019-09-04  5:42   ` Oliver O'Halloran
2019-09-04 11:22     ` Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 17/23] powerpc/pci: Fix crash with enabled movable BARs Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 18/23] powerpc/pci: Handle BAR movement Sergey Miroshnichenko
2019-09-04  5:37   ` Oliver O'Halloran
2019-09-06 16:24     ` Sergey Miroshnichenko
2019-09-09 14:02       ` Oliver O'Halloran
2019-08-16 16:50 ` [PATCH v5 19/23] PCI: hotplug: Configure MPS for hot-added bridges during bus rescan Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 20/23] PCI: hotplug: movable BARs: Enable the feature by default Sergey Miroshnichenko
2019-08-16 16:50 ` [PATCH v5 21/23] nvme-pci: Handle movable BARs Sergey Miroshnichenko
2019-08-16 16:51 ` [PATCH v5 22/23] PCI/portdrv: Declare support of " Sergey Miroshnichenko
2019-08-16 16:51 ` [PATCH v5 23/23] PCI: pciehp: movable BARs: Trigger a domain rescan on hp events Sergey Miroshnichenko

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20191015221449.GA181069@google.com \
    --to=helgaas@kernel.org \
    --cc=David.Laight@ACULAB.COM \
    --cc=linux-pci@vger.kernel.org \
    --cc=linux@yadro.com \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=lukas@wunner.de \
    --cc=oohall@gmail.com \
    --cc=rajatja@google.com \
    --cc=s.miroshnichenko@yadro.com \
    --cc=sbobroff@linux.ibm.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-PCI Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-pci/0 linux-pci/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-pci linux-pci/ https://lore.kernel.org/linux-pci \
		linux-pci@vger.kernel.org
	public-inbox-index linux-pci

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-pci


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git