From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-by2on0095.outbound.protection.outlook.com ([207.46.100.95]:49632 "EHLO na01-by2-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932079AbcANSyz (ORCPT ); Thu, 14 Jan 2016 13:54:55 -0500 Message-ID: <5697EEF8.30406@caviumnetworks.com> Date: Thu, 14 Jan 2016 10:54:48 -0800 From: David Daney MIME-Version: 1.0 To: Alex Williamson CC: , , , Subject: Re: [RFC PATCH] pci: Identify Enhanced Allocation (EA) BAR Equivalent resources References: <20160114172645.23429.9938.stgit@gimli.home> In-Reply-To: <20160114172645.23429.9938.stgit@gimli.home> Content-Type: text/plain; charset="UTF-8"; format=flowed Sender: linux-pci-owner@vger.kernel.org List-ID: On 01/14/2016 09:26 AM, 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? Is the flag exposed to userspace in any way? I haven't dug into what uses the flags. One problem we have seen is with the lspci utility which cannot distinguish between SROIV BARs and EA provisioned BARs. Would, or could, this be used there? David Daney > > Signed-off-by: Alex Williamson > --- > 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) \ >