Re: [PATCH v4] vfio-pci: Allow to mmap sub-page MMIO BARs if the mmio page is exclusive
From: Yongji Xie
Date: Tue Jun 28 2016 - 06:10:24 EST
Hi, Alex
On 2016/6/25 0:43, Alex Williamson wrote:
On Fri, 24 Jun 2016 23:37:02 +0800
Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx> wrote:
Hi, Alex
On 2016/6/24 11:37, Alex Williamson wrote:
On Fri, 24 Jun 2016 10:52:58 +0800
Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx> wrote:
On 2016/6/24 0:12, Alex Williamson wrote:
On Mon, 30 May 2016 21:06:37 +0800
Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx> wrote:
+static void vfio_pci_probe_mmaps(struct vfio_pci_device *vdev)
+{
+ struct resource *res;
+ int bar;
+ struct vfio_pci_dummy_resource *dummy_res;
+
+ INIT_LIST_HEAD(&vdev->dummy_resources_list);
+
+ for (bar = PCI_STD_RESOURCES; bar <= PCI_STD_RESOURCE_END; bar++) {
+ res = vdev->pdev->resource + bar;
+
+ if (!IS_ENABLED(CONFIG_VFIO_PCI_MMAP))
+ goto no_mmap;
+
+ if (!(res->flags & IORESOURCE_MEM))
+ goto no_mmap;
+
+ /*
+ * The PCI core shouldn't set up a resource with a
+ * type but zero size. But there may be bugs that
+ * cause us to do that.
+ */
+ if (!resource_size(res))
+ goto no_mmap;
+
+ if (resource_size(res) >= PAGE_SIZE) {
+ vdev->bar_mmap_supported[bar] = true;
+ continue;
+ }
+
+ if (!(res->start & ~PAGE_MASK)) {
+ /*
+ * Add a dummy resource to reserve the remainder
+ * of the exclusive page in case that hot-add
+ * device's bar is assigned into it.
+ */
+ dummy_res = kzalloc(sizeof(*dummy_res), GFP_KERNEL);
+ if (dummy_res == NULL)
+ goto no_mmap;
+
+ dummy_res->resource.start = res->end + 1;
+ dummy_res->resource.end = res->start + PAGE_SIZE - 1;
+ dummy_res->resource.flags = res->flags;
+ if (request_resource(res->parent,
+ &dummy_res->resource)) {
+ kfree(dummy_res);
+ goto no_mmap;
+ }
Isn't it true that request_resource() only tells us that at a given
point in time, no other drivers have reserved that resource? It seems
like it does not guarantee that the resource isn't routed to another
device or that another driver won't at some point attempt to request
that same resource. So for example if a user constructs their initrd
to bind vfio-pci to devices before other modules load, this
request_resource() may succeed, at the expense of drivers loaded later
now failing. The behavior will depend on driver load order and we're
not actually insuring that the overflow resource is unused, just that
we got it first. Can we do better? Am I missing something that
prevents this? Thanks,
Alex
Couldn't PCI resources allocator prevent this, which will find a
empty slot in the resource tree firstly, then try to request that
resource in allocate_resource() when a PCI device is probed.
And I'd like to know why a PCI device driver would attempt to
call request_resource()? Should this be done in PCI enumeration?
Hi Yongji,
Looks like most pci drivers call pci_request_regions(). From there the
call path is:
pci_request_selected_regions
__pci_request_selected_regions
__pci_request_region
__request_mem_region
__request_region
__request_resource
We see this driver ordering issue sometimes with users attempting to
blacklist native pci drivers, trying to leave a device free for use by
vfio-pci. If the device is a graphics card, the generic vesa or uefi
driver can request device resources causing a failure when vfio-pci
tries to request those same resources. I expect that unless it's a
boot device, like vga in my example, the resources are not enabled
until the driver opens the device, therefore the request_resource() call
doesn't occur until that point.
For another trivial example, look at /proc/iomem as you load and unload
a driver, on my laptop with e1000e unloaded I see:
e1200000-e121ffff : 0000:00:19.0
e123e000-e123efff : 0000:00:19.0
When e1000e is loaded, each of these becomes claimed by the e1000e
driver:
e1200000-e121ffff : 0000:00:19.0
e1200000-e121ffff : e1000e
e123e000-e123efff : 0000:00:19.0
e123e000-e123efff : e1000e
Clearly pci core knows the resource is associated with the device, but
I don't think we're tapping into that with request_resource(), we're
just potentially stealing resources that another driver might have
claimed otherwise as I described above. That's my suspicion at
least, feel free to show otherwise if it's incorrect. Thanks,
Alex
Thanks for your explanation. But I still have one question.
Shouldn't PCI core have claimed all PCI device's resources
after probing those devices. If so, request_resource() will fail
when vfio-pci try to steal resources that another driver might
request later. Anything I missed here? Some device resources
would not be claimed in PCI core?
Hi Yongji,
I don't know what to say, this is not how the interface currently
works. request_resource() is a driver level interface that tries to
prevent drivers from claiming conflicting resources. In this patch
you're trying to use it to probe whether a resource maps to another
device. This is not what it does. request_resource() will happily let
you claim any resource you want, so long as nobody else claimed it
first. So the only case where the assumptions in this patch are valid
is if we can guarantee that any potentially conflicting device has a
driver loaded that has claimed those resources. As it is here,
vfio-pci will happily attempt to request resources that might overlap
with another device and might break drivers that haven't yet had a
chance to probe their devices. I don't think that's acceptable.
Thanks,
Alex
I'm trying to get your point. Let me give an example here.
I'm not sure whether my understanding is right. Please
point it out if I'm wrong.
We assume that there are two PCI devices like this:
240000000000-24feffffffff : /pciex@3fffe40400000
240000000000-2400ffffffff : PCI Bus 0002:01
240000000000-240000007fff : 0002:01:00.0
240000000000-240000007fff : vfio-pci
240000008000-24000000ffff : 0002:01:01.0
240000008000-24000000ffff : lpfc
Do you mean vfio-pci driver will succeed in requesting
dummy_res: [240000008000-24000000ffff] (PAGE_SIZE is 64K)
if it is loaded before lpfc driver? Like this:
240000000000-24feffffffff : /pciex@3fffe40400000
240000000000-2400ffffffff : PCI Bus 0002:01
240000000000-240000007fff : 0002:01:00.0
240000000000-240000007fff : vfio-pci
240000008000-24000000ffff : 0002:01:01.0
240000008000-24000000ffff : <BAD> --> vfio-pci call
request_resource()
Then lpfc driver will fail when it attempts to call
pci_request_regions() later.
But is it possible that the dummy_res become the child of
the res: 0002:01:01.0? Wouldn't request_resource() fail when
it found parent res: PCI Bus 0002:01 already have conflict
child res: 0002:01:01.0.
And I think the case that request_resource() will succeed
should like this:
240000000000-24feffffffff : /pciex@3fffe40400000
240000000000-2400ffffffff : PCI Bus 0002:01
240000000000-240000007fff : 0002:01:00.0
240000010000-240000017fff : 0002:01:01.0
There is a mem hole: [240000008000-24000000ffff] after
PCI probing. After loading drivers, the resources tree
will be:
240000000000-24feffffffff : /pciex@3fffe40400000
240000000000-2400ffffffff : PCI Bus 0002:01
240000000000-240000007fff : 0002:01:00.0
240000000000-240000007fff : vfio-pci
240000008000-24000000ffff : <BAD> ---> vfio-pci call
request_resource()
240000010000-240000017fff : 0002:01:01.0
240000010000-240000017fff : lpfc
Thanks,
Yongji