Message-ID: <20160629205352.286838d8@t450s.home>
Date:	Wed, 29 Jun 2016 20:53:52 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Yongji Xie <xyjxie@...ux.vnet.ibm.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	bhelgaas@...gle.com, aik@...abs.ru, benh@...nel.crashing.org,
	paulus@...ba.org, mpe@...erman.id.au, warrier@...ux.vnet.ibm.com,
	zhong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	gwshan@...ux.vnet.ibm.com, kevin.tian@...el.com
Subject: Re: [PATCH v4] vfio-pci: Allow to mmap sub-page MMIO BARs if the
 mmio page is exclusive

On Thu, 30 Jun 2016 10:40:23 +0800
Yongji Xie <xyjxie@...ux.vnet.ibm.com> wrote:

> Hi Alex,
> 
> On 2016/6/30 4:03, Alex Williamson wrote:
> 
> > On Tue, 28 Jun 2016 13:47:23 -0600
> > Alex Williamson <alex.williamson@...hat.com> wrote:
> >  
> >> On Tue, 28 Jun 2016 18:09:46 +0800
> >> Yongji Xie <xyjxie@...ux.vnet.ibm.com> wrote:
> >>  
> >>> Hi, Alex
> >>>
> >>> On 2016/6/25 0:43, Alex Williamson wrote:
> >>>      
> >>>> On Fri, 24 Jun 2016 23:37:02 +0800
> >>>> Yongji Xie <xyjxie@...ux.vnet.ibm.com> wrote:
> >>>>       
> >>>>> Hi, Alex
> >>>>>
> >>>>> On 2016/6/24 11:37, Alex Williamson wrote:
> >>>>>       
> >>>>>> On Fri, 24 Jun 2016 10:52:58 +0800
> >>>>>> Yongji Xie <xyjxie@...ux.vnet.ibm.com> wrote:  
> >>>>>>> On 2016/6/24 0:12, Alex Williamson wrote:  
> >>>>>>>> On Mon, 30 May 2016 21:06:37 +0800
> >>>>>>>> Yongji Xie <xyjxie@...ux.vnet.ibm.com> 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@...fe40400000
> >>>     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@...fe40400000
> >>>     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@...fe40400000
> >>>     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@...fe40400000
> >>>     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,  
> 
> Yeah, I also noticed that:-). Now I'm trying to find a proper
> name. What do you think about "vfio-pci (dummy)"?

How about "vfio sub-page reserved"?  Thanks,

Alex

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