Date:   Wed, 2 Nov 2016 17:15:23 +0100
From:   Auger Eric <eric.auger@...hat.com>
To:     Robin Murphy <robin.murphy@....com>,
        Will Deacon <will.deacon@....com>
Cc:     yehuday@...vell.com, alex.williamson@...hat.com,
        jason@...edaemon.net, kvm@...r.kernel.org, marc.zyngier@....com,
        joro@...tes.org, Punit Agrawal <punit.agrawal@....com>,
        p.fedin@...sung.com, drjones@...hat.com,
        linux-kernel@...r.kernel.org, Bharat.Bhushan@...escale.com,
        Jean-Philippe.Brucker@....com, iommu@...ts.linux-foundation.org,
        pranav.sawargaonkar@...il.com, christoffer.dall@...aro.org,
        tglx@...utronix.de, Manish.Jaggi@...iumnetworks.com,
        linux-arm-kernel@...ts.infradead.org, eric.auger.pro@...il.com
Subject: Re: [PATCH v14 00/16] KVM PCIe/MSI passthrough on ARM/ARM64

Hi Robin,
On 24/10/2016 21:39, Robin Murphy wrote:
> On 21/10/16 10:26, Auger Eric wrote:
>> Hi Will,
>>
>> On 20/10/2016 19:32, Will Deacon wrote:
>>> Hi Eric,
>>>
>>> Thanks for posting this.
>>>
>>> On Wed, Oct 12, 2016 at 01:22:08PM +0000, Eric Auger wrote:
>>>> This is the second respin on top of Robin's series [1], addressing Alex' comments.
>>>>
>>>> Major changes are:
>>>> - MSI-doorbell API now is moved to DMA IOMMU API following Alex suggestion
>>>>   to put all API pieces at the same place (so eventually in the IOMMU
>>>>   subsystem)
>>>> - new iommu_domain_msi_resv struct and accessor through DOMAIN_ATTR_MSI_RESV
>>>>   domain with mirror VFIO capability
>>>> - more robustness I think in the VFIO layer
>>>> - added "iommu/iova: fix __alloc_and_insert_iova_range" since with the current
>>>>   code I failed allocating an IOVA page in a single page domain with upper part
>>>>   reserved
>>>>
>>>> IOVA range exclusion will be handled in a separate series
>>>>
>>>> The priority really is to discuss and freeze the API and especially the MSI
>>>> doorbell's handling. Do we agree to put that in DMA IOMMU?
>>>>
>>>> Note: the size computation does not take into account possible page overlaps
>>>> between doorbells but it would add quite a lot of complexity i think.
>>>>
>>>> Tested on AMD Overdrive (single GICv2m frame) with I350 VF assignment.
>>>
>>> Marc, Robin and I sat down and had a look at the series and, whilst it's
>>> certainly addressing a problem that we desperately want to see fixed, we
>>> think that it's slightly over-engineering in places and could probably
>>> be simplified in the interest of getting something upstream that can be
>>> used as a base, on which the ABI can be extended as concrete use-cases
>>> become clear.
>>>
>>> Stepping back a minute, we're trying to reserve some of the VFIO virtual
>>> address space so that it can be used by devices to map their MSI doorbells
>>> using the SMMU. With your patches, this requires that (a) the kernel
>>> tells userspace about the size and alignment of the doorbell region
>>> (MSI_RESV) and (b) userspace tells the kernel the VA-range that can be
>>> used (RESERVED_MSI_IOVA).
>>>
>>> However, this is all special-cased for MSI doorbells and there are
>>> potentially other regions of the VFIO address space that are reserved
>>> and need to be communicated to userspace as well. We already know of
>>> hardware where the PCI RC intercepts p2p accesses before they make it
>>> to the SMMU, and other hardware where the MSI doorbell is at a fixed
>>> address. This means that we need a mechanism to communicate *fixed*
>>> regions of virtual address space that are reserved by VFIO. I don't
>>> even particularly care if VFIO_MAP_DMA enforces that, but we do need
>>> a way to tell userspace "hey, you don't want to put memory here because
>>> it won't work well with devices".
>>
>> I think we all agree on this. Exposing an API to the user space
>> reporting *fixed* reserved IOVA ranges is a requirement anyway. The
>> problem was quite clearly stated by Alex in
>> http://lkml.iu.edu/hypermail/linux/kernel/1610.0/03308.html
>> (VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE)
>>
>> I started working on this VFIO_IOMMU_TYPE1_INFO_CAP_IOVA_RANGE
>> capability but to me and I think according to Alex, it was a different
>> API from MSI_RESV.
>>
>>>
>>> In that case, we end up with something like your MSI_RESV capability,
>>> but actually specifying a virtual address range that is simply not to
>>> be used by MAP_DMA -- we don't say anything about MSIs. Now, taking this
>>> to its logical conclusion, we no longer need to distinguish between
>>> remappable reserved regions and fixed reserved regions in the ABI.
>>> Instead, we can have the kernel allocate the virtual address space for
>>> the remappable reserved regions (probably somewhere in the bottom 4GB)
>>> and expose them via the capability.
>>
>>
>> If I understand correctly you want the host to arbitrarily choose where
>> it puts the iovas reserved for MSI and not ask the userspace.
>>
>> Well so we are back to the discussions we had in Dec 2015 (see Marc's
>> answer in http://thread.gmane.org/gmane.comp.emulators.kvm.arm.devel/3858).
> 
> To an extent, yes, however the difference is that we now know we
> definitely have to deal with situations in which userspace *cannot* be
> in total control of the memory map, and that changes the game:
> 
>     _________
>    /         \
>   /  Fixed    \
>  /  things (A) \
> (   _________   )
>  \ /   MSI   \ /
>   X doorbells X
>  / \___(B)___/ \
> (               )
>  \ Remappable  /
>   \ things (C)/
>    \_________/
> 
> In the absence of A, then B == C so it was very hard to not want to
> implement C. As soon as A *has* to be implemented for other reasons,
> then that is also sufficient to encompass B. C can still be implemented
> later as a nice-to-have, but is not necessary to get B off the ground.
> 
>> - So I guess you will init an iova_domain seomewhere below the 4GB to
>> allocate the MSIs. what size are you going to choose. Don't you have the
>> same need to dimension the iova range.
>> - we still need to assess the MSI assignment safety. How will we compute
>> safety for VFIO?
> 
> Absolutely. We're talking in general terms of the userspace ABI here,
> although that can't help but colour the underlying implementation
> decisions.

Sorry for the delay I was out of the office last week.

The userspace ABI to retrieve reserved regions is the *easy* part. It is
based on VFIO capability chain and I have an RFC ready.

 Of course the VFIO internals still have to handle the
> specific case of MSIs, but that's basically no more than this:
> 
>   static bool msi_isolation = true; /* until proven otherwise */
>   static unsigned long msi_remap_virt_base = 0x08000000; /* fits QEMU */
>   static size_t msi_remap_size;
> 
>   vfio_msi_thing_callback(thing) {
>   	msi_remap_size += thing->info.size;
>   	msi_isolation &= thing->info.flags & PROVIDES_ISOLATION;
>   }
> 
>   vfio_msi_init(...) {
>   	...
>   #ifdef CONFIG_X86
>   	msi_remap_virt_base = 0xfee00000;
>   	msi_remap_size = 0x100000;
>   	msi_isolation = irq_remapping_enabled;
>   #else
>   	irq_for_each_msi_thing(vfio_msi_thing_callback);
>   #endif
>   	...
>   }
> 
>   vfio_attach_group(...) {
>   	...
>   	if (!msi_isolation && !allow_unsafe_interrupts)
>   		return -ENOWAY;
>   	...
>   	get_msi_region_cookie(domain, msi_remap_base, msi_remap_size);
>   	...
>   }
I doubt Alex will accept to put that code in VFIO. He suggested in the
past to use the IOMMU API to retrieve the reserved region(s).

what about adding a reserved_regions list in iommu_domain and add a new
iommu_ops, something like
void add_reserved_regions(struct iommu_domain *, struct device *dev)
whose role would be to populate the list. This add_reserved_regions()
would be called on __iommu_attach_device. The list would be emptied on
iommu_domain_free().

arm-smmu cb implementation would be in charge of
- computing non ACS PCI host bridge windows from @dev,
- computing msi_rebase_map/size computation

on x86, cb would simply populate the MSI window.

vfio would lookup the iommu domain reserved_regions list on
VFIO_IOMMU_GET_INFO

Drawback of this approach is the security aspect is not handled by the
IOMMU API.

Note that combining v14 series and this one would implement everything I
think + giving the flexibility for the userspace to choose where it put
things. But well, LPC discussions will bring the last word obviously.
> 
> And when a well-behaved userspace queries the reserved regions, that
> base address and size is just one of potentially several that it should
> get back. It's that "querying the reserved regions" bit that needs to be
> gotten right first time.
> 
> Note that at this point I'm no longer even overly bothered about the
> details of irq_for_each_msi_thing(), as it's an internal kernel
> interface and thus malleable, although obviously the simpler the better.
> I have to say Punit's idea of iterating irq_domains does actually look
> really neat and tidy as a proof-of-concept, and also makes me think off
> on a tangent that it would be sweet to be able to retrieve base+size
> from dev->msi_domain to pre-allocate MSI pages in default domains, and
> obviate the compose 'failure' case.

As Punit mentionned, the natural place where the msi doorbell base,
size and irq_remapping can be retrieved looks to be the irqchip itself.
It works perfectly fine for v2m and its. Hence my first attempt to use a
cb at this level (irqchip msi_doorbell_info up to v11).

Adding a cb at irq_domain level looks quite impractical to me to
retrieve the info. Actually I don't see how to manage that without
adding new fields in irq_domain struct. If you have any suggestion,
please let me know.

Thanks

Eric

> 
>>  This simplifies things in the
>>> following ways:
>>>
>>>   * You don't need to keep track of MSI vs DMA addresses in the VFIO rbtree
>> right: I guess you rely on iommu_map to return an error in case the iova
>> is already mapped somewhere else.
>>>   * You don't need to try collapsing doorbells into a single region
>> why? at host level I guess you will init a single iova domain?
> 
> Yeah, right now this one goes either way - as things stand, it does make
> life easier on the host side to make a single region to hang off the
> back of the current iova_cookie magic, and as illustrated above it's
> possibly the most trivial part of the whole thing, but the point is we
> still don't *need* to. Since a userspace ABI for generic reservations
> has to be able handle more than one region for the sake of non-MSI
> things, we'd be free to change the kernel-side implementation in future
> to just report multiple doorbells as individual regions - for starters,
> if and when we add dynamic reservations and userspace gets to pick its
> own IOVAs for those, it'll be a damn sight easier *not* to coalesce
> everything.
> 
>>>   * You don't need a special MAP flavour to map MSI doorbells
>> right
>>>   * The ABI is reusable for PCI p2p and fixed doorbells
>> right
>>
>> Aren't we moving the issue at user-space? Currently QEMU mach-virt
>> address space is fully static. Adapting mach-virt to adjust to host
>> constraints is not straightforward. It is simple to reject the
>> assignment in case of collision but more difficult to react positively.
> 
> The point is that we *have* to move at least some of the issue to
> userspace, and by then I'm struggling to see any real difference between
> these situations:
> 
> a) QEMU asks VFIO to map some pages for DMA, gets an error back because
> VFIO detects it conflicts with a reserved region, and gives up.
> b) QEMU starts by asking VFIO what regions are reserved, realises they
> will overlap with its hard-coded RAM address, and gives up.
> 
> where (a) requires a bunch of kernel machinery to second-guess
> userspace, while (b) simply relies on userspace not being broken. And if
> userspace fails at not being broken, then we simply retain the behaviour
> which actually happens right now:
> 
> c) QEMU maps some pages for DMA at the same address as PCI config space
> on the underlying hardware. Hilarity ensues.
> 
> Of course, userspace could be anything other than QEMU as well, so it's
> not necessarily second-guessable at all; maybe we make the arbitrary
> msi_remap_virt_base a VFIO module parameter to be more accommodating.
> Who knows, maybe it turns out that's enough to keep users happy and we
> never need to implement fully dynamic reservations.
> 
> Robin.
> 
>>> I really think it would make your patch series both generally useful and
>>> an awful lot smaller, whilst leaving the door open to ABI extension on
>>> a case-by-case basis when we determine that it's really needed.
>>
>> I would like to have a better understanding of how you assess the
>> security and dimension the iova domain. This is the purpose of msi
>> doorbell registration, which is not neat at all I acknowledge but well I
>> did not find any other solution and did not get any other suggestion.
>> Besides I think the per-cpu thing is over-engineered and this can
>> definitively be simplified.
>>
>> VFIO part was reviewed by Alex and I don't have the impression that this
>> is the blocking part. besides there is on iova.c fix,
>> IOMMU_CAP_INTR_REMAP removal; so is it really over-complicated?
>>
>> Thanks
>>
>> Eric
>>
>>>
>>> Thoughts?
>>>
>>> Will
>>>
>>> _______________________________________________
>>> linux-arm-kernel mailing list
>>> linux-arm-kernel@...ts.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>>>
>>
> 
> 
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