From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga11.intel.com ([192.55.52.93]:10222 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759297AbcAURwh (ORCPT ); Thu, 21 Jan 2016 12:52:37 -0500 Date: Thu, 21 Jan 2016 09:48:59 -0800 From: "Sean O. Stalley" To: Alex Williamson Cc: bhelgaas@google.com, linux-pci@vger.kernel.org, david.daney@cavium.com Subject: Re: [RFC PATCH] pci: Identify Enhanced Allocation (EA) BAR Equivalent resources Message-ID: <20160121174858.GA3064@sean.stalley.intel.com> References: <20160114172645.23429.9938.stgit@gimli.home> <1453321227.32741.332.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 In-Reply-To: <1453321227.32741.332.camel@redhat.com> Sender: linux-pci-owner@vger.kernel.org List-ID: On Wed, Jan 20, 2016 at 01:20:27PM -0700, Alex Williamson wrote: > 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 > > --- > > > 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 > > Thanks Alex, I'll pick it up and fix the lspci case. -Sean > >  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) \ > > >