On Thu, 2016-01-14 at 10:26 -0700, Alex Williamson wrote:
> We've done a pretty good job of abstracting EA from drivers, but there
> are some properties of BAR Equivalent resources that don't really jive
> with traditional PCI BARs. In particular, natural alignment is only
> encouraged, not required.
>
> Why does this matter? There are drivers like vfio-pci that will
> happily gobble up the EA abstraction that's been implemented and
> expose a device using EA to userspace as if those resources are
> traditional BARs. Pretty cool. The vfio API is bus agnostic, so it
> doesn't care about alignment. The problem comes with PCI config space
> emulation where we don't let userspace manipulate the BAR value, but
> we do emulate BAR sizing. The abstraction kind of falls apart if
> userspace gets garbage when they try to size what appears to be a
> traditional BAR, but is actually a BAR equivalent.
>
> We could simply round up the size in vfio to make it naturally
> aligned, but then we're imposing artificial sizes to the user and we
> have the discontinuity that BAR size emulation and vfio region size
> reporting don't agree on the size. I think what we want to do is
> expose EA to the user, reporting traditional BARs with BEIs as
> zero-sized and providing additional regions for the user to access
> each EA region, whether it has a BEI or not.
>
> To facilitate that, a flag indicating whether a PCI resource is a
> traditional BAR or BAR equivalent seems much nicer than attempting
> to size the BAR ourselves or deducing it through the EA capability.
>
> Thoughts?
>
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
Just to loop back on this, it seems like we do have some support and
use cases beyond what I proposed. Thanks for the discussion of that.
However, I'm reluctant to post this formally because the change is user
visible, it consumes a limited resource, and I don't know how quickly
vfio-pci is going to be able to make use of this flag. The vfio-pci
work may not happen until a device appears with poorly sized resources
that has some use case with vfio-pci. Even then, we may be able to
infer the BEI association without this flag. So, while I'm not opposed
to this flag, I don't see a need to drive it right now and those that
do have a more immediate need are welcome to take over. Thanks,
Alex
> drivers/pci/pci.c | 2 +-
> include/linux/ioport.h | 2 ++
> 2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 314db8c..174c734 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -2229,7 +2229,7 @@ void pci_pm_init(struct pci_dev *dev)
>
> static unsigned long pci_ea_flags(struct pci_dev *dev, u8 prop)
> {
> - unsigned long flags = IORESOURCE_PCI_FIXED;
> + unsigned long flags = IORESOURCE_PCI_FIXED | IORESOURCE_PCI_EA_BEI;
>
> switch (prop) {
> case PCI_EA_P_MEM:
> diff --git a/include/linux/ioport.h b/include/linux/ioport.h
> index 24bea08..5acc194 100644
> --- a/include/linux/ioport.h
> +++ b/include/linux/ioport.h
> @@ -105,6 +105,8 @@ struct resource {
> /* PCI control bits. Shares IORESOURCE_BITS with above PCI ROM. */
> #define IORESOURCE_PCI_FIXED (1<<4) /* Do not move resource */
>
> +/* PCI Enhanced Allocation defined BAR equivalent resource */
> +#define IORESOURCE_PCI_EA_BEI (1<<5)
>
> /* helpers to define resources */
> #define DEFINE_RES_NAMED(_start, _size, _name, _flags) \
>
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