From: Gerald Schaefer <gerald.schaefer@de.ibm.com>
To: Joerg Roedel <joro@8bytes.org>
Cc: Sebastian Ott <sebott@linux.vnet.ibm.com>,
iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 0/2] iommu/s390: Fix iommu-groups and add sysfs support
Date: Fri, 28 Apr 2017 14:46:34 +0200 [thread overview]
Message-ID: <20170428144634.7950c8cf@thinkpad> (raw)
In-Reply-To: <20170427210325.GE1332@8bytes.org>
Hi Joerg,
I guess we are a bit special on s390 (again), see below. Sebastian is more
familiar with the base s390 PCI code, he may correct me if I'm wrong.
On Thu, 27 Apr 2017 23:03:25 +0200
Joerg Roedel <joro@8bytes.org> wrote:
> > Well, there is a separate zpci_dev for each pci_dev on s390,
> > and each of those has its own separate dma-table (thus not shared).
>
> Is that true for all functions of a PCIe card, so does every function of
> a device has its own zpci_dev structure and thus its own DMA-table?
Yes, clp_add_pci_device() is called for every function, which in turn calls
zpci_create_device() with a freshly allocated zdev. zpci_enable_device()
then sets up a new DMA address space for each function.
> My assumption came from the fact that the zpci_dev is read from
> pci_dev->sysdata, which is propagated there from the pci_bridge
> through the pci_root_bus structures.
The zdev gets there via zpci_create_device() -> zpci_scan_bus() ->
pci_scan_root_bus(), which is done for every single function.
Not sure if I understand this right, but it looks like we set up a new PCI
bus for each function.
> > Given this "separate zpci_dev for each pci_dev" situation, I don't
> > see what this update actually changes, compared to the previous code,
> > see also my comments to that patch.
>
> The add_device call-back is invoked for every function of a pci-device,
> because each function gets its own pci_dev structure. Also we usually
> group all functions of a PCI-device together into one iommu-group,
> because we don't trust that the device isolates its functions from each
> other.
OK, but similar to the add_device callback, zpci_create_device() is
also invoked for every function. So, allocating a new iommu-group in
zpci_create_device() will never lead to any group sharing.
I am however a bit confused now, about how we would have allowed group
sharing with the current s390 IOMMU code, or IOW in which scenario would
iommu_group_get() in the add_device callback find a shareable iommu-group?
In the attach_dev callback, we provide the option to "force" multiple
functions using the same iommu-domain / DMA address space, by de-registering
the per-function DMA address space and registering a common space. But
such functions would only be in the same iommu "domain" and not "group",
if I get this right.
So, I guess we may have an issue with not sharing iommu-groups when
it could make sense to do so. But your patch would not fix this, as
we still would allocate separate iommu-groups for all functions.
Regards,
Gerald
next prev parent reply other threads:[~2017-04-28 12:46 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-27 15:28 [RFC PATCH 0/2] iommu/s390: Fix iommu-groups and add sysfs support Joerg Roedel
2017-04-27 15:28 ` [PATCH 1/2] iommu/s390: Fix IOMMU groups Joerg Roedel
2017-04-27 18:11 ` Gerald Schaefer
2017-04-27 21:12 ` Joerg Roedel
2017-04-28 13:20 ` Gerald Schaefer
2017-04-28 14:40 ` Joerg Roedel
2017-04-28 17:50 ` kbuild test robot
2017-04-27 15:28 ` [PATCH 2/2] iommu/s390: Add support for iommu_device handling Joerg Roedel
2017-04-28 23:02 ` kbuild test robot
2017-04-27 18:10 ` [RFC PATCH 0/2] iommu/s390: Fix iommu-groups and add sysfs support Gerald Schaefer
2017-04-27 21:03 ` Joerg Roedel
2017-04-28 12:46 ` Gerald Schaefer [this message]
2017-04-28 14:55 ` Joerg Roedel
2017-04-28 15:25 ` Sebastian Ott
2017-04-28 22:29 ` Joerg Roedel
2017-04-28 18:06 ` Gerald Schaefer
2017-04-28 22:40 ` Joerg Roedel
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