Subject: Re: [PATCH v5 3/4] PCI: Add a new option for resource_alignment to reassign alignment

From: Bjorn Helgaas <[email-masked]>
Date: Thu, 29 Sep 2016 06:54:23 -0500

On Thu, Sep 29, 2016 at 10:38:01AM +0800, Yongji Xie wrote:
> On 2016/9/29 6:42, Bjorn Helgaas wrote:
> 
> >On Tue, Sep 13, 2016 at 05:00:33PM +0800, Yongji Xie wrote:
> >>When using resource_alignment kernel parameter, the current
> >>implement reassigns the alignment by changing resources' size
> >>which can potentially break some drivers. For example, the driver
> >>uses the size to locate some register whose length is related
> >>to the size.
> >>
> >>This patch adds a new option "noresize" for the parameter to
> >>solve this problem.
> >>
> >>Signed-off-by: Yongji Xie <[email-masked]>
> >>---
> >>  Documentation/kernel-parameters.txt |    9 ++++++---
> >>  drivers/pci/pci.c                   |   37 +++++++++++++++++++++++++----------
> >>  2 files changed, 33 insertions(+), 13 deletions(-)
> >>
> >>diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> >>index a4f4d69..d6a340d 100644
> >>--- a/Documentation/kernel-parameters.txt
> >>+++ b/Documentation/kernel-parameters.txt
> >>@@ -3023,9 +3023,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >>  				window. The default value is 64 megabytes.
> >>  		resource_alignment=
> >>  				Format:
> >>-				[<order of align>@][<domain>:]<bus>:<slot>.<func>[; ...]
> >>-				[<order of align>@]pci:<vendor>:<device>\
> >>-						[:<subvendor>:<subdevice>][; ...]
> >>+				[<order of align>@][noresize@][<domain>:]
> >>+				<bus>:<slot>.<func>[; ...]
> >>+				[<order of align>@][noresize@]pci:<vendor>:<device>
> >>+				[:<subvendor>:<subdevice>][; ...]
> >>  				Specifies alignment and device to reassign
> >>  				aligned memory resources.
> >>  				If <order of align> is not specified,
> >>@@ -3036,6 +3037,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >>  				instances of a device, the PCI vendor,
> >>  				device, subvendor, and subdevice may be
> >>  				specified, e.g., 4096@pci:8086:9c22:103c:198f
> >>+				noresize: Don't change the resources' sizes when
> >>+				reassigning alignment.
> >>  		ecrc=		Enable/disable PCIe ECRC (transaction layer
> >>  				end-to-end CRC checking).
> >>  				bios: Use BIOS/firmware settings. This is the
> >>diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> >>index b8357d7..37f8062 100644
> >>--- a/drivers/pci/pci.c
> >>+++ b/drivers/pci/pci.c
> >>@@ -4946,11 +4946,13 @@ static DEFINE_SPINLOCK(resource_alignment_lock);
> >>  /**
> >>   * pci_specified_resource_alignment - get resource alignment specified by user.
> >>   * @dev: the PCI device to get
> >>+ * @resize: whether or not to change resources' size when reassigning alignment
> >>   *
> >>   * RETURNS: Resource alignment if it is specified.
> >>   *          Zero if it is not specified.
> >>   */
> >>-static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev)
> >>+static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
> >>+		bool *resize)
> >>  {
> >>  	int seg, bus, slot, func, align_order, count;
> >>  	unsigned short vendor, device, subsystem_vendor, subsystem_device;
> >>@@ -4974,6 +4976,13 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev)
> >>  		} else {
> >>  			align_order = -1;
> >>  		}
> >>+
> >>+		if (!strncmp(p, "noresize@", 9)) {
> >>+			*resize = false;
> >>+			p += 9;
> >>+		} else
> >>+			*resize = true;
> >>+
> >>  		if (strncmp(p, "pci:", 4) == 0) {
> >>  			/* PCI vendor/device (subvendor/subdevice) ids are specified */
> >>  			p += 4;
> >>@@ -5045,6 +5054,7 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
> >>  {
> >>  	int i;
> >>  	struct resource *r;
> >>+	bool resize = true;
> >>  	resource_size_t align, size;
> >>  	u16 command;
> >>@@ -5058,7 +5068,7 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
> >>  		return;
> >>  	/* check if specified PCI is target device to reassign */
> >>-	align = pci_specified_resource_alignment(dev);
> >>+	align = pci_specified_resource_alignment(dev, &resize);
> >>  	if (!align)
> >>  		return;
> >>@@ -5086,15 +5096,22 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
> >>  		}
> >>  		size = resource_size(r);
> >>-		if (size < align) {
> >>-			size = align;
> >>-			dev_info(&dev->dev,
> >>-				"Rounding up size of resource #%d to %#llx.\n",
> >>-				i, (unsigned long long)size);
> >>+		if (resize) {
> >>+			if (size < align) {
> >>+				size = align;
> >>+				dev_info(&dev->dev,
> >>+					"Rounding up size of resource #%d to %#llx.\n",
> >>+					i, (unsigned long long)size);
> >>+			}
> >>+			r->flags |= IORESOURCE_UNSET;
> >>+			r->end = size - 1;
> >>+			r->start = 0;
> >Why do you want to keep this code that changes the size of the resource?
> >Can't we just *always* use IORESOURCE_STARTALIGN as below?  It seems like
> >that would potentially fix bugs, as you mention.
> >
> >I think it'd be better if we didn't change the size, even if the user
> >didn't specify "noresize@".  We wouldn't even need the "noresize@" option
> >then.
> >
> >Or is there some reason to keep the resize?  If it's just a question of
> >being afraid to change the existing behavior because of the risk, I'm
> >willing to take that risk.
> 
> Hi Bjorn,
> 
> Thanks for your time. The reason is just like what you said.  I'm worried
> that this may break the existing behavior.  I'll updated this patch as
> you suggested.

I guess one problem is that if we *don't* change the size, there's
nothing to prevent something else from being allocated right after the
resource.  For example, if we have a 1K BAR, and the objective is to
make sure it's on a 4K page by itself, not only do we have to align
the BAR on 4K, we *also* have to keep anything else from using the
remaining 3K of that page.

Maybe it's easier to fix the drivers so they don't rely on the size.
Do you have examples of such drivers?

I'm hesitant to add a new option to the "pci=resource_alignment"
parameter because it's already very complicated and it exposes more
kernel internals than I'm really comfortable with.  If we can figure
out a way to make the existing parameter work for everybody, I would
prefer that.

Bjorn

> >>+		} else {
> >>+			r->flags &= ~IORESOURCE_SIZEALIGN;
> >>+			r->flags |= IORESOURCE_STARTALIGN | IORESOURCE_UNSET;
> >>+			r->start = max(align, size);
> >>+			r->end = r->start + size - 1;
> >>  		}
> >>-		r->flags |= IORESOURCE_UNSET;
> >>-		r->end = size - 1;
> >>-		r->start = 0;
> >>  	}
> >>  	/* Need to disable bridge's resource window,
> >>  	 * to enable the kernel to reassign new resource
> >>-- 
> >>1.7.9.5
> >>
> >>--
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