LKML Archive on
help / color / mirror / Atom feed
From: Alex Williamson <>
To: Bjorn Helgaas <>
Cc: Doug Meyer <>,
	Logan Gunthorpe <>,,,
	linux-ntb <>,
	Bjorn Helgaas <>, Jon Mason <>,
	"Jiang, Dave" <>,
	Allen Hubbe <>,
Subject: Re: [PATCH 2/2] NTB: PCI Quirk to Enable Switchtec NT Functionality with IOMMU On
Date: Tue, 22 May 2018 16:13:05 -0600	[thread overview]
Message-ID: <20180522161305.04ec9817@w520.home> (raw)
In-Reply-To: <>

On Tue, 22 May 2018 16:51:26 -0500
Bjorn Helgaas <> wrote:

> [+cc Alex]
> On Tue, May 22, 2018 at 02:09:59PM -0700, Doug Meyer wrote:
> > Logan answered the questions quite thoroughly. (Thanks, Logan!)  
> When you repost it, please rework the commit log so it answers the
> questions directly.  Otherwise the next reader may have the same
> questions again.  E.g., say something about how the proxy IDs are not
> programmable and are fixed in the hardware so all we have to do is
> read them.
> I don't think the question of when the aliases need to be added is
> quite closed.  Logan said "it seems pci_add_dma_alias() must be called
> before the driver is initialized and therefore in a quirk", but that
> doesn't make clear *why* the alias needs to be added before the driver
> is initialized.  The alias shouldn't be needed until the device does a
> DMA, and it shouldn't do that until after the driver initializes.

Aliases for devices that don't have a representation on the bus is only
one use for pci_add_dma_alias(), we can also use it when the aliased
device is visible on the bus and then it factors not only into the IOMMU
context entries for a given device, but also the grouping of multiple
devices that must be done without a host endpoint driver.
> I suspect the reason the existing quirks are in drivers/pci/quirks.c
> is because the IOMMU driver is in the host OS, but the host may not
> have a driver for the device if the device is passed through to a
> guest OS.  In that case, the only way to add the alias is by using a
> quirk that is always built into the host OS.
> We could argue that the driver in the guest should be able to tell the
> host's IOMMU driver about these aliases, but I doubt there's an
> interface for that.

Sounds like a dangerous interface, imagine two physical functions on a
device, each assigned to separate guests where one guest could usurp
context entries for hidden devices from the other guest.  Thanks,


  reply	other threads:[~2018-05-22 22:13 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-17 12:00 [PATCH 0/2] PCI Quirk Patchset for Microsemi Switchtec NTB dmeyer
2018-05-17 12:00 ` [PATCH 1/2] NTB: Migrate PCI Constants to Cannonical PCI Header dmeyer
2018-05-17 13:25   ` Bjorn Helgaas
2018-05-17 12:00 ` [PATCH 2/2] NTB: PCI Quirk to Enable Switchtec NT Functionality with IOMMU On dmeyer
2018-05-17 13:45   ` Bjorn Helgaas
2018-05-17 16:06     ` Logan Gunthorpe
     [not found]       ` <>
2018-05-22 21:51         ` Bjorn Helgaas
2018-05-22 22:13           ` Alex Williamson [this message]
2018-05-22 22:23           ` Logan Gunthorpe
2018-05-23 13:33             ` Bjorn Helgaas
2018-05-23 20:21               ` Logan Gunthorpe
2018-05-17 15:48   ` Logan Gunthorpe
2018-05-22 21:08     ` Doug Meyer

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20180522161305.04ec9817@w520.home \ \ \ \ \ \ \ \ \ \ \ \ \
    --subject='Re: [PATCH 2/2] NTB: PCI Quirk to Enable Switchtec NT Functionality with IOMMU On' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).