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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: Mon, 26 Feb 2018 19:36:34 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 19/02/18 13:46, Alexey Kardashevskiy wrote:
> On 16/02/18 16:28, David Gibson wrote:
>> On Wed, Feb 14, 2018 at 08:55:41AM -0700, Alex Williamson wrote:
>>> On Wed, 14 Feb 2018 19:09:16 +1100
>>> Alexey Kardashevskiy <address@hidden> wrote:
>>>> 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,
>>> It is possible for x86 systems to have translation applied to MMIO vs
>>> RAM such that the processor view and device view of MMIO are different,
>>> putting them into separate address spaces, but this is not typical and
>>> not available on the class of chipsets that QEMU emulates for PC.
>>> Therefore I think it's correct that MMIO and RAM all live in one big
>>> happy, flat address space as they do now (assuming the guest IOMMU is
>>> not present or disabled).  Thanks,
>> Ah.. I think the thing I was missing is that on PC (at least with
>> typical chipsets) *all* MMIO essentially comes from PCI space.  Even
>> the legacy devices are essentially ISA which is treated as being on a
>> bridge under the PCI space.  On non-x86 there are often at least a
>> handful of MMIO devices that aren't within the PCI space - say, the
>> PCI host bridge itself at least.  x86 avoids that by using the
>> separate IO space, which is also a bit weird in that it's
>> simultaneously direct attached to the cpu (and so doesn't need bridge
>> configuration), but also identified with the legay IO space treated as
>> being under two bridges (host->PCI, PCI->ISA) for other purposes.
>> Maybe it's just me, but I find it makes more sense to me if I think of
>> it as the two ASes being equal because on PC the system address map
>> (address_space_memory) is made equal to the PCI address map, rather
>> than the PCI address map being made equal to the system one.
> It makes more sense to me too, we just do not exploit/expose this on SPAPR
> - the PCI address space only has 2xIOMMU and 1xMSI windows and that's it,
> no MMIO which is mapped to the system AS only (adding those to the PCI AS
> is little tricky as mentioned elsewhere).
> Anyway, I still wonder - what needs to be addressed in the 2/4 patch in
> order to proceed?



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