[PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory

Lorenzo Pieralisi lpieralisi at kernel.org
Thu Dec 7 06:50:09 PST 2023


On Thu, Dec 07, 2023 at 09:38:25AM -0400, Jason Gunthorpe wrote:
> On Thu, Dec 07, 2023 at 11:13:52AM +0100, Lorenzo Pieralisi wrote:
> > > > What about the other way around - would we have a prefetchable BAR that
> > > > has portions which are unprefetchable?
> > > 
> > > I would say possibly.
> > > 
> > > Prefetch is a dead concept in PCIe, it was obsoleted in PCI-X about 20
> > > years ago. No PCIe system has ever done prefetch.
> > > 
> > > There is a strong incentive to mark BAR's as prefetchable because it
> > > allows 64 bit addressing in configurations with bridges.
> > 
> > If by strong incentive you mean the "Additional guidance on the
> > Prefetchable Bit in Memory Space BARs" in the PCI express specifications,
> > I think it has been removed from the spec and the criteria that had to be
> > met to implement it were basically impossible to fulfill on ARM systems,
> > it did not make any sense in the first place.
> 
> No, I mean many systems don't have room to accommodate large 32 bit
> BARs and the only real way to make stuff work is to have a 64 bit BAR
> by setting prefetchable.

That's what the implementation note I mentioned referred to ;)

> Given mis-marking a read-side-effect region as prefetchable has no
> actual consequence on PCI-E I would not be surprised to learn people
> have done this.

PCIe specs 6.1, 7.5.1.2.1 "Base Address Registers"

"A function is permitted to mark a range as prefetchable if there are
no side effects on reads..."

I don't think that an OS should use the prefetchable flag to infer
anything (even though we do at the moment -> sysfs mappings, I know that
the prefetchable concept is being scrapped from the PCI specs altogether
for a reason), I don't see though how we can say that's a SW bug at
present given what I quoted above.

I'd agree that it is best not to use that flag for new code we are adding
(because it will be deprecated soon).

Thanks,
Lorenzo



More information about the linux-arm-kernel mailing list