Re: [PATCH v4] vfio-pci: Allow to mmap sub-page MMIO BARs if the mmio page is exclusive

From: Alex Williamson
Date: Wed Jun 29 2016 - 16:03:22 EST

On Tue, 28 Jun 2016 13:47:23 -0600
Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:

> On Tue, 28 Jun 2016 18:09:46 +0800
> Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx> wrote:
>
> > 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.
>
> Yes, that is my supposition.
>
> > 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
>
> Ok, let's stop guessing about this. I modified your patch as follows
> so I could easily test it on a 4k host:
>
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -110,6 +110,9 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
> return (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA;
> }
>
> +#define VFIO_64K_PAGE_SIZE (64*1024)
> +#define VFIO_64K_PAGE_MASK (~(VFIO_64K_PAGE_SIZE-1))
> +
> static void vfio_pci_probe_mmaps(struct vfio_pci_device *vdev)
> {
> struct resource *res;
> @@ -135,12 +138,13 @@ static void vfio_pci_probe_mmaps(struct vfio_pci_device *vdev)
> if (!resource_size(res))
> goto no_mmap;
>
> - if (resource_size(res) >= PAGE_SIZE) {
> + if (resource_size(res) >= VFIO_64K_PAGE_SIZE) {
> vdev->bar_mmap_supported[bar] = true;
> continue;
> }
>
> - if (!(res->start & ~PAGE_MASK)) {
> + if (!(res->start & ~VFIO_64K_PAGE_MASK)) {
> + int ret;
> /*
> * Add a dummy resource to reserve the remainder
> * of the exclusive page in case that hot-add
> @@ -151,10 +155,12 @@ static void vfio_pci_probe_mmaps(struct vfio_pci_device *vdev)
> goto no_mmap;
>
> dummy_res->resource.start = res->end + 1;
> - dummy_res->resource.end = res->start + PAGE_SIZE - 1;
> + dummy_res->resource.end = res->start + VFIO_64K_PAGE_SIZE - 1;
> dummy_res->resource.flags = res->flags;
> - if (request_resource(res->parent,
> - &dummy_res->resource)) {
> + ret = request_resource(res->parent,
> + &dummy_res->resource);
> + if (ret) {
> +dev_info(&vdev->pdev->dev, "Failed to request_resource %lx-%lx (%d)\n", dummy_res->resource.start, dummy_res->resource.end, ret);
> kfree(dummy_res);
> goto no_mmap;
> }
>
> IOW, enforce 64k for mmap regardless of PAGE_SIZE. Then I find a
> system configuration to test it:
>
> ee400000-ef4fffff : PCI Bus 0000:07
> ef480000-ef49ffff : 0000:07:00.0
> ef480000-ef483fff : 0000:08:10.0
> ef484000-ef487fff : 0000:08:10.2
> ef488000-ef48bfff : 0000:08:10.4
> ef48c000-ef48ffff : 0000:08:10.6
> ef490000-ef493fff : 0000:08:11.0
> ef494000-ef497fff : 0000:08:11.2
> ef498000-ef49bfff : 0000:08:11.4
> ef4a0000-ef4bffff : 0000:07:00.0
> ef4a0000-ef4a3fff : 0000:08:10.0
> ef4a4000-ef4a7fff : 0000:08:10.2
> ef4a8000-ef4abfff : 0000:08:10.4
> ef4ac000-ef4affff : 0000:08:10.6
> ef4b0000-ef4b3fff : 0000:08:11.0
> ef4b4000-ef4b7fff : 0000:08:11.2
> ef4b8000-ef4bbfff : 0000:08:11.4
>
> This is an 82576 NIC where each VF has two 16k BARs (0 & 3), where all
> the VF BAR0s are in a contiguous range and all the VF BAR3s are in a
> separate contiguous range. The igbvf driver is not loaded, so the
> other resources are free to be claimed, what happens?
>
> It looks like you're right, the request_resource() fails with:
>
> vfio-pci 0000:08:10.0: Failed to request_resource ef4a4000-ef4affff (-16)
> vfio-pci 0000:08:10.0: Failed to request_resource ef484000-ef48ffff (-16)
>
> So we get back -EBUSY which means we hit a conflict. I would have
> expected that this means our res->parent that we're using for
> request_resource() is only, for instance, ef480000-ef483fff (ie. the
> BAR itself) thus our request for ef484000-ef48ffff exceeds the end of
> the parent, but adding the parent resource range to the dev_info(), it
> actually shows the range being ef480000-ef49ffff, so the parent is
> actually the 07:00.0 resource. In fact, we can't even use
> request_resource() like this to claim the BAR itself, which is why we
> use pci_request_selected_regions(), which allows conflicts, putting the
> requested resource at the leaf of the tree.
>
> So I guess I retract this concern about the use of request_resource(),
> it does seem to behave as intended. Unless I can spot anything else or
> other reviewers have comments, I'll queue this into my next branch for
> v4.8. Thanks,


Ok, one more test, I found that I have access to the following USB
devices:

00:1a.0 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Enhanced Host Controller #2 (rev 05) (prog-if 20 [EHCI])
Region 0: Memory at f7a08000 (32-bit, non-prefetchable) [size=1K]

00:1d.0 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Enhanced Host Controller #1 (rev 05) (prog-if 20 [EHCI])
Region 0: Memory at f7a07000 (32-bit, non-prefetchable) [size=1K]

These are nicely mapped such that vfio-pci can claim the residual space
from the page, which results in the following in /proc/iomem:

f7a07000-f7a073ff : 0000:00:1d.0
f7a07000-f7a073ff : vfio
f7a07400-f7a07fff : <BAD>
f7a08000-f7a083ff : 0000:00:1a.0
f7a08000-f7a083ff : vfio
f7a08400-f7a08fff : <BAD>

I should have looked more closely at your previous reply, I didn't
think that "<BAD>" was literally the owner of these dummy resources.
Please fix this to report something that isn't going frighten users
and make small children cry. Thanks,

Alex