Re: [Qemu-devel] [PATCH qemu v7 2/4] vfio/pci: Relax DMA map errors for MMIO regions

[Alexey Kardashevskiy]

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?

On x86 it listens on the "memory" AS, on spapr - on the
"address@hidden" AS, this will just create more confusion over time...



-- 
Alexey

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