Re: [RFC PATCH] PCI: Tolerate non-prefetchable 64-bit BARs in prefetchable windows

[Ilpo Järvinen]

On Thu, 10 Sep 2026, Ard Biesheuvel wrote:

> 
> On Thu, 10 Sep 2026, at 19:31, Ilpo Järvinen wrote:
> > On Thu, 10 Sep 2026, Ard Biesheuvel wrote:
> >
> >> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> >> 
> >> The prefetchable vs. non-prefetchable distinction is a relic of
> >> conventional PCI, to denote from which regions PCI-PCI bridges were
> >> permitted to perform speculative readahead.
> >> 
> >> For software compatibility reasons, PCI Express inherited the Type 1
> >> header and models PCIe root ports as PCI-PCI bridges. However, this
> >> readahead behavior does not exist in PCIe, and so this distinction has
> >> mostly become meaningless on the bridge level.
> >> 
> >> As per the PCIe r6.3 ECN "Removing Prefetchable Terminology", the
> >> 'prefetchable' designation has been removed from the specification
> >> entirely, on the basis that it is obsolete, and is being abused to
> >> inform memory mapping attributes and other device/BAR level properties
> >> that it was never intended for.
> >> 
> >> Given the limited range for non-prefetchable windows in the Type 1
> >> header, and the fact that the distinction no longer exists for PCIe,
> >> resource allocation performed by firmware may result in non-prefetchable
> >> 64-bit BARs being allocated inside prefetchable bridge windows.
> >> 
> >> Linux rejects such allocations ("can't claim; no compatible bridge
> >> window") when it encounters them, but will usually fail to produce an
> >> alternative allocation, given that firmware wouldn't have placed them
> >> there in the first place if there was sufficient space in the
> >> non-prefetchable window.
> >> 
> >> So at the very least, let's not reject such allocations when they were
> >> made by the firmware.
> >> 
> >> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> >> Cc: "Ilpo Järvinen" <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> >> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> >> ---
> >> Link: https://github.com/tianocore/edk2/issues/13104
> >> 
> >>  drivers/pci/pci.c   | 3 ++-
> >>  include/linux/pci.h | 4 ++--
> >>  2 files changed, 4 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> >> index b2879a6be5f8..e33eb9f3a139 100644
> >> --- a/drivers/pci/pci.c
> >> +++ b/drivers/pci/pci.c
> >> @@ -761,7 +761,8 @@ struct resource *pci_find_parent_resource(const struct pci_dev *dev,
> >>  			 * not, the allocator made a mistake.
> >>  			 */
> >>  			if (r->flags & IORESOURCE_PREFETCH &&
> >> -			    !(res->flags & IORESOURCE_PREFETCH))
> >> +			    !(res->flags & IORESOURCE_PREFETCH) &&
> >> +			    !pci_is_pcie(dev))
> >>  				return NULL;
> >>  
> >>  			/*
> >
> > I was more thinking along the lines of always setting IORESOURCE_PREFETCH 
> > for 64-bit BARs on PCIe devices but I've not had time to look at that/test 
> > how many things would break as a result. ...It would seem much simpler 
> > solution to differentiate PCI from PCIe while keeping the existing logic 
> > without adding similar pci_is_pcie() checks everywhere.
> >
> 
> I agree that the code changes would be much simpler.
> 
> However, would this impact the PCI metadata observed by all consumers,
> including userspace,

Yes, it will impact userspace. But quoting you from above:

"is being abused to inform memory mapping attributes and other device/BAR 
level properties that it was never intended for."

What does the userspace then do with the information? Does it qualify 
under "it was never intended for"?

> and drivers that may expect a certain BAR layout,

??? Would that even be spec compliant??

> and/or base decisions about memory attributes on this?

The point is to consider them 64-bit window eligible so yes, kernel would 
definitely be basing decision on that but that's intentional.

> In particular, I am concerned about non-prefetchable BARs that actually
> have side effects on read, being mapped with WC (or Normal-NC on arm64)
> semantics, where the interconnect may widen, combine or reorder accesses.

So on a more concrete terms, you're referring to the check in 
proc_bus_pci_mmap()? And the one in __pci_resource_attr_is_visible() + 
pci_dev_resource_wc_is_visible()? I suppose that wouldn't work then.

So if just setting IORESOURCE_PREFETCH is not workable, how about adding a 
getter for res->flags which adds IORESOURCE_PREFETCH into the returned 
flags if it's PCIe device and (in the end) use the raw value only in those 
places that actually care about wc distinction. What I don't want to see 
us adding that pci_is_pcie() everywhere.

-- 
 i.