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Re: [Qemu-ppc] [PATCH qemu v7 2/4] vfio/pci: Relax DMA map errors for MM

From: Alexey Kardashevskiy
Subject: Re: [Qemu-ppc] [PATCH qemu v7 2/4] vfio/pci: Relax DMA map errors for MMIO regions
Date: Wed, 14 Feb 2018 19:09:16 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 14/02/18 12:33, David Gibson wrote:
> On Tue, Feb 13, 2018 at 07:20:56PM +1100, Alexey Kardashevskiy wrote:
>> On 13/02/18 16:41, David Gibson wrote:
>>> On Tue, Feb 13, 2018 at 04:36:30PM +1100, David Gibson wrote:
>>>> On Tue, Feb 13, 2018 at 12:15:52PM +1100, Alexey Kardashevskiy wrote:
>>>>> On 13/02/18 03:06, Alex Williamson wrote:
>>>>>> On Mon, 12 Feb 2018 18:05:54 +1100
>>>>>> Alexey Kardashevskiy <address@hidden> wrote:
>>>>>>> On 12/02/18 16:19, David Gibson wrote:
>>>>>>>> On Fri, Feb 09, 2018 at 06:55:01PM +1100, Alexey Kardashevskiy wrote:  
>>>>>>>>> At the moment if vfio_memory_listener is registered in the system 
>>>>>>>>> memory
>>>>>>>>> address space, it maps/unmaps every RAM memory region for DMA.
>>>>>>>>> It expects system page size aligned memory sections so vfio_dma_map
>>>>>>>>> would not fail and so far this has been the case. A mapping failure
>>>>>>>>> would be fatal. A side effect of such behavior is that some MMIO pages
>>>>>>>>> would not be mapped silently.
>>>>>>>>> However we are going to change MSIX BAR handling so we will end having
>>>>>>>>> non-aligned sections in vfio_memory_listener (more details is in
>>>>>>>>> the next patch) and vfio_dma_map will exit QEMU.
>>>>>>>>> In order to avoid fatal failures on what previously was not a failure 
>>>>>>>>> and
>>>>>>>>> was just silently ignored, this checks the section alignment to
>>>>>>>>> the smallest supported IOMMU page size and prints an error if not 
>>>>>>>>> aligned;
>>>>>>>>> it also prints an error if vfio_dma_map failed despite the page size 
>>>>>>>>> check.
>>>>>>>>> Both errors are not fatal; only MMIO RAM regions are checked
>>>>>>>>> (aka "RAM device" regions).
>>>>>>>>> If the amount of errors printed is overwhelming, the MSIX relocation
>>>>>>>>> could be used to avoid excessive error output.
>>>>>>>>> This is unlikely to cause any behavioral change.
>>>>>>>>> Signed-off-by: Alexey Kardashevskiy <address@hidden>  
>>>>>>>> There are some relatively superficial problems noted below.
>>>>>>>> But more fundamentally, this feels like it's extending an existing
>>>>>>>> hack past the point of usefulness.
>>>>>>>> The explicit check for is_ram_device() here has always bothered me -
>>>>>>>> it's not like a real bus bridge magically knows whether a target
>>>>>>>> address maps to RAM or not.
>>>>>>>> What I think is really going on is that even for systems without an
>>>>>>>> IOMMU, it's not really true to say that the PCI address space maps
>>>>>>>> directly onto address_space_memory.  Instead, there's a large, but
>>>>>>>> much less than 2^64 sized, "upstream window" at address 0 on the PCI
>>>>>>>> bus, which is identity mapped to the system bus.  Details will vary
>>>>>>>> with the system, but in practice we expect nothing but RAM to be in
>>>>>>>> that window.  Addresses not within that window won't be mapped to the
>>>>>>>> system bus but will just be broadcast on the PCI bus and might be
>>>>>>>> picked up as a p2p transaction.  
>>>>>>> Currently this p2p works only via the IOMMU, direct p2p is not possible 
>>>>>>> as
>>>>>>> the guest needs to know physical MMIO addresses to make p2p work and it
>>>>>>> does not.
>>>>>> /me points to the Direct Translated P2P section of the ACS spec, though
>>>>>> it's as prone to spoofing by the device as ATS.  In any case, p2p
>>>>>> reflected from the IOMMU is still p2p and offloads the CPU even if
>>>>>> bandwidth suffers vs bare metal depending on if the data doubles back
>>>>>> over any links.  Thanks,
>>>>> Sure, I was just saying that p2p via IOMMU won't be as simple as broadcast
>>>>> on the PCI bus, IOMMU needs to be programmed in advance to make this work,
>>>>> and current that broadcast won't work for the passed through devices.
>>>> Well, sure, p2p in a guest with passthrough devices clearly needs to
>>>> be translated through the IOMMU (and p2p from a passthrough to an
>>>> emulated device is essentially impossible).
>>>> But.. what does that have to do with this code.  This is the memory
>>>> area watcher, looking for memory regions being mapped directly into
>>>> the PCI space.  NOT IOMMU regions, since those are handled separately
>>>> by wiring up the IOMMU notifier.  This will only trigger if RAM-like,
>>>> non-RAM regions are put into PCI space *not* behind an IOMMMU.
>>> Duh, sorry, realised I was mixing up host and guest IOMMU.  I guess
>>> the point here is that this will map RAM-like devices into the host
>>> IOMMU when there is no guest IOMMU, allowing p2p transactions between
>>> passthrough devices (though not from passthrough to emulated devices).
>> Correct.
>>> The conditions still seem kind of awkward to me, but I guess it makes
>>> sense.
>> Is it the time to split this listener to RAM-listener and PCI bus listener?
> I'm not really sure what you mean by that.
>> On x86 it listens on the "memory" AS, on spapr - on the
>> "address@hidden" AS, this will just create more confusion over time...
> Hm, it's still logically the same AS in each case - the PCI address
> space.  It's just that in the x86 case that happens to be the same as
> the memory AS (at least when there isn't a guest side IOMMU).

Hm. Ok.

> I do wonder if that's really right even for x86 without IOMMU.  The
> PCI address space is identity mapped to the "main" memory address
> space there, but I'm not sure it's mapped th *all* of the main memory

What should have been in the place of "th" above? :)

> address space, probably just certain ranges of it.  That's what I was
> suggesting earlier in the thread.

afaict vfio is trying to mmap every RAM MR == KVM memory slot, no ranges or
anything like that. I am trying to figure out what is not correct with the
patch. Thanks,


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