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

[Auger Eric]

Hi Alex,

On 13/03/18 17:56, Alex Williamson wrote:
> [Cc +Eric]
> 
> On Tue, 13 Mar 2018 15:53:19 +1100
> Alexey Kardashevskiy <address@hidden> wrote:
> 
>> On 7/3/18 1:17 pm, Alexey Kardashevskiy wrote:
>>> On 26/02/18 19:36, Alexey Kardashevskiy wrote:  
>>>> 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?  
>>>>
>>>> Ping?
>>>>  
>>>
>>>
>>> Ping?
>>>
>>>   
>>
>>
>> Could anyone please enlighten me what needs to be done with this patchset
>> to proceed, or confirm that it is ok but not going anywhere as everybody is
>> busy with other things? :) Thanks,
> 
> Actually making sure it compiles would have been nice:
> 
> qemu.git/hw/vfio/common.c: In function ‘vfio_listener_region_add’:
> qemu.git/hw/vfio/common.c:550:40: error: invalid operands to binary & (have 
> ‘Int128 {aka struct Int128}’ and ‘hwaddr {aka long long unsigned int}’)
>          if ((iova & pgmask) || (llsize & pgmask)) {
>                                         ^
> qemu.git/hw/vfio/common.c: In function ‘vfio_listener_region_del’:
> qemu.git/hw/vfio/common.c:669:50: error: invalid operands to binary & (have 
> ‘Int128 {aka struct Int128}’ and ‘hwaddr {aka long long unsigned int}’)
>          try_unmap = !((iova & pgmask) || (llsize & pgmask));
>                                                   ^
> Clearly llsize needs to be wrapped in int128_get64() here.  Should I
> presume that testing of this patch is equally lacking?  Eric, have you
> done any testing of this?  I think this was fixing a mapping issue you
> reported.  Thanks,
not that version unfortunately. I don't have access to the HW on which I
had the issue anymore. Maybe I could by the beg of next week?

Thanks

Eric
> 
> Alex
>