Re: [PATCH V1 18/32] osdep: import MADV_DOEXEC

[Steven Sistare <steven.sistare@oracle.com>]

On 10/15/2020 4:36 PM, Alex Williamson wrote:
> On Thu, 8 Oct 2020 12:32:35 -0400
> Steven Sistare <steven.sistare@oracle.com> wrote:
>> On 8/24/2020 6:30 PM, Alex Williamson wrote:
>>> On Wed, 19 Aug 2020 17:52:26 -0400
>>> Steven Sistare <steven.sistare@oracle.com> wrote:  
>>>> On 8/17/2020 10:42 PM, Alex Williamson wrote:  
>>>>> On Mon, 17 Aug 2020 15:44:03 -0600
>>>>> Alex Williamson <alex.williamson@redhat.com> wrote:  
>>>>>> On Mon, 17 Aug 2020 17:20:57 -0400
>>>>>> Steven Sistare <steven.sistare@oracle.com> wrote:  
>>>>>>> On 8/17/2020 4:48 PM, Alex Williamson wrote:      
>>>>>>>> On Mon, 17 Aug 2020 14:30:51 -0400
>>>>>>>> Steven Sistare <steven.sistare@oracle.com> wrote:
>>>>>>>>         
>>>>>>>>> On 7/30/2020 11:14 AM, Steve Sistare wrote:        
>>>>>>>>>> Anonymous memory segments used by the guest are preserved across a re-exec
>>>>>>>>>> of qemu, mapped at the same VA, via a proposed madvise(MADV_DOEXEC) option
>>>>>>>>>> in the Linux kernel. For the madvise patches, see:
>>>>>>>>>>
>>>>>>>>>> https://lore.kernel.org/lkml/1595869887-23307-1-git-send-email-anthony.yznaga@oracle.com/
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>>>>>>>>>> ---
>>>>>>>>>>  include/qemu/osdep.h | 7 +++++++
>>>>>>>>>>  1 file changed, 7 insertions(+)          
>>>>>>>>>
>>>>>>>>> Hi Alex,
>>>>>>>>>   The MADV_DOEXEC functionality, which is a pre-requisite for the entire qemu 
>>>>>>>>> live update series, is getting a chilly reception on lkml.  We could instead 
>>>>>>>>> create guest memory using memfd_create and preserve the fd across exec.  However, 
>>>>>>>>> the subsequent mmap(fd) will return a different VA than was used previously, 
>>>>>>>>> which  is a problem for memory that was registered with vfio, as the original VA 
>>>>>>>>> is remembered in the kernel struct vfio_dma and used in various kernel functions, 
>>>>>>>>> such as vfio_iommu_replay.
>>>>>>>>>
>>>>>>>>> To fix, we could provide a VFIO_IOMMU_REMAP_DMA ioctl taking iova, size, and
>>>>>>>>> new_vaddr.  The implementation finds an exact match for (iova, size) and replaces 
>>>>>>>>> vaddr with new_vaddr.  Flags cannot be changed.
>>>>>>>>>
>>>>>>>>> memfd_create plus VFIO_IOMMU_REMAP_DMA would replace MADV_DOEXEC.
>>>>>>>>> vfio on any form of shared memory (shm, dax, etc) could also be preserved across
>>>>>>>>> exec with shmat/mmap plus VFIO_IOMMU_REMAP_DMA.
>>>>>>>>>
>>>>>>>>> What do you think        
>>>>>>>>
>>>>>>>> Your new REMAP ioctl would have parameters identical to the MAP_DMA
>>>>>>>> ioctl, so I think we should just use one of the flag bits on the
>>>>>>>> existing MAP_DMA ioctl for this variant.        
>>>>>>>
>>>>>>> Sounds good.
>>>>>>>       
>>>>>>>> Reading through the discussion on the kernel side there seems to be
>>>>>>>> some confusion around why vfio needs the vaddr beyond the user call to
>>>>>>>> MAP_DMA though.  Originally this was used to test for virtually
>>>>>>>> contiguous mappings for merging and splitting purposes.  This is
>>>>>>>> defunct in the v2 interface, however the vaddr is now used largely for
>>>>>>>> mdev devices.  If an mdev device is not backed by an IOMMU device and
>>>>>>>> does not share a container with an IOMMU device, then a user MAP_DMA
>>>>>>>> ioctl essentially just registers the translation within the vfio
>>>>>>>> container.  The mdev vendor driver can then later either request pages
>>>>>>>> to be pinned for device DMA or can perform copy_to/from_user() to
>>>>>>>> simulate DMA via the CPU.
>>>>>>>>
>>>>>>>> Therefore I don't see that there's a simple re-architecture of the vfio
>>>>>>>> IOMMU backend that could drop vaddr use.          
>>>>>>>
>>>>>>> Yes.  I did not explain on lkml as you do here (thanks), but I reached the 
>>>>>>> same conclusion.
>>>>>>>       
>>>>>>>> I'm a bit concerned this new
>>>>>>>> remap proposal also raises the question of how do we prevent userspace
>>>>>>>> remapping vaddrs racing with asynchronous kernel use of the previous
>>>>>>>> vaddrs.          
>>>>>>>
>>>>>>> Agreed.  After a quick glance at the code, holding iommu->lock during 
>>>>>>> remap might be sufficient, but it needs more study.      
>>>>>>
>>>>>> Unless you're suggesting an extended hold of the lock across the entire
>>>>>> re-exec of QEMU, that's only going to prevent a race between a remap
>>>>>> and a vendor driver pin or access, the time between the previous vaddr
>>>>>> becoming invalid and the remap is unprotected.    
>>>>
>>>> OK.  What if we exclude mediated devices?  Its appears they are the only
>>>> ones where the kernel may async'ly use the vaddr, via call chains ending in 
>>>> vfio_iommu_type1_pin_pages or vfio_iommu_type1_dma_rw_chunk.
>>>>
>>>> The other functions that use dma->vaddr are
>>>>     vfio_dma_do_map 
>>>>     vfio_pin_map_dma 
>>>>     vfio_iommu_replay 
>>>>     vfio_pin_pages_remote
>>>> and they are all initiated via userland ioctl (if I have traced all the code 
>>>> paths correctly).  Thus iommu->lock would protect them.
>>>>
>>>> We would block live update in qemu if the config includes a mediated device.
>>>>
>>>> VFIO_IOMMU_REMAP_DMA would return EINVAL if the container has a mediated device.  
>>>
>>> That's not a solution I'd really be in favor of.  We're eliminating an
>>> entire class of devices because they _might_ make use of these
>>> interfaces, but anyone can add a vfio bus driver, even exposing the
>>> same device API, and maybe make use of some of these interfaces in that
>>> driver.  Maybe we'd even have reason to do it in vfio-pci if we had
>>> reason to virtualize some aspect of a device.  I think we're setting
>>> ourselves up for a very complicated support scenario if we just
>>> arbitrarily decide to deny drivers using certain interfaces.
>>>   
>>>>>>>> Are we expecting guest drivers/agents to quiesce the device,
>>>>>>>> or maybe relying on clearing bus-master, for PCI devices, to halt DMA?        
>>>>>>>
>>>>>>> No.  We want to support any guest, and the guest is not aware that qemu
>>>>>>> live update is occurring.
>>>>>>>       
>>>>>>>> The vfio migration interface we've developed does have a mechanism to
>>>>>>>> stop a device, would we need to use this here?  If we do have a
>>>>>>>> mechanism to quiesce the device, is the only reason we're not unmapping
>>>>>>>> everything and remapping it into the new address space the latency in
>>>>>>>> performing that operation?  Thanks,        
>>>>>>>
>>>>>>> Same answer - we don't require that the guest has vfio migration support.      
>>>>>>
>>>>>> QEMU toggling the runstate of the device via the vfio migration
>>>>>> interface could be done transparently to the guest, but if your
>>>>>> intention is to support any device (where none currently support the
>>>>>> migration interface) perhaps it's a moot point.      
>>>>
>>>> That sounds useful when devices support.  Can you give me some function names
>>>> or references so I can study this qemu-based "vfio migration interface".  
>>>
>>> The uAPI is documented in commit a8a24f3f6e38.  We're still waiting on
>>> the QEMU support or implementation in an mdev vendor driver.
>>> Essentially migration exposes a new region of the device which would be
>>> implemented by the vendor driver.  A register within that region
>>> manipulates the device state, so a device could be stopped by clearing
>>> the 'run' bit in that register.
>>>   
>>>>>> It seems like this
>>>>>> scheme only works with IOMMU backed devices where the device can
>>>>>> continue to operate against pinned pages, anything that might need to
>>>>>> dynamically pin pages against the process vaddr as it's running async
>>>>>> to the QEMU re-exec needs to be blocked or stopped.  Thanks,    
>>>>
>>>> Yes, true of this remap proposal.
>>>>
>>>> I wanted to unconditionally support all devices, which is why I think that
>>>>
>>>> MADV_DOEXEC is a nifty solution.  If you agree, please add your voice to the
>>>> lkml discussion.  
>>
>> Hi Alex, here is a modified proposal to remap vaddr in the face of async requests
>> from mediated device drivers.
>>
>> Define a new flag VFIO_DMA_MAP_FLAG_REMAP for use with VFIO_IOMMU_UNMAP_DMA and
>> VFIO_IOMMU_MAP_DMA.
>>
>> VFIO_IOMMU_UNMAP_DMA  flags=VFIO_DMA_MAP_FLAG_REMAP
>>   Discard vaddr on the existing dma region defined by (iova, size), but keep the
>>   struct vfio_dma.  Subsequent translation requests are blocked.
>>   The implementation sets a flag in struct vfio_dma.  vfio_pin_pages() and
>>   vfio_dma_rw() acquire iommu->lock, check the flag, and retry.
>>   Called before exec.
>>
>> VFIO_IOMMU_MAP_DMA  flags=VFIO_DMA_MAP_FLAG_REMAP
>>   Remap the region (iova, size) to vaddr, and resume translation requests.
>>   Called after exec.
>>
>> Unfortunately, remap as defined above has an undesirable side effect.  The mdev
>> driver may use kernel worker threads which serve requests from multiple clients
>> (eg i915/gvt workload_thread).  A process that fails to call MAP_DMA with REMAP,
>> or is tardy doing so, will delay other processes who are stuck waiting in
>> vfio_pin_pages or vfio_dma_rw.  This is unacceptable, and I mention this scheme in
>> case I am misinterpreting the code (maybe they do not share a single struct vfio_iommu
>> instance?), or in case you see a way to salvage it.
> 
> Right, that's my first thought when I hear that the pin and dma_rw paths
> are blocked as well, we cannot rely on userspace to unblock anything.
> A malicious user may hold out just to see how long until the host
> becomes unusable.  Userspace determines how many groups share a
> vfio_iommu.
> 
>> Here is a more robust implementation.  It only works for dma regions backed by
>> a file, such as memfd or shm.
>>
>> VFIO_IOMMU_UNMAP_DMA  flags=VFIO_DMA_MAP_FLAG_REMAP
>>   Find the file and offset for iova, and save the struct file pointer in
>>   struct vfio_dma.  In vfio_pin_pages and vfio_dma_rw and their descendants,
>>   if file* is set, then call pagecache_get_page() to get the pfn, instead of
>>   get_user_pages.
>>
>> VFIO_IOMMU_MAP_DMA  flags=VFIO_DMA_MAP_FLAG_REMAP
>>   Remap the region (iova, size) to vaddr and drop the file reference.
>>
>> This begs the question of whether we can always use pagecache_get_page, and
>> eliminate the dependency on vaddr.  The translation performance could be
>> different, though.
>>
>> I have not implemented this yet.  Any thoughts before I do?
> 
> That's a pretty hefty usage restriction, but what I take from it is
> that these are mechanisms which provide a fallback lookup path that can
> service callers in the interim during the gap of the range being
> remapped.  The callers are always providing an IOVA and wishing to do
> something to the memory referenced by that IOVA, we just need a
> translation mechanism.  The IOMMU itself is also such an alternative
> lookup, via iommu_iova_to_phys(), but of course requiring an
> IOMMU-backed device is just another usage restriction, potentially one
> that's not even apparent to the user.
> 
> Is a more general solution to make sure there's always an IOVA-to-phys
> lookup mechanism available, implementing one if not provided by the
> IOMMU or memory backing interface?  We'd need to adapt the dma_rw
> interface to work on either a VA or PA, and pinning pages on
> UNMAP+REMAP, plus stashing them in a translation structure, plus
> dynamically adapting to changes (ex. the IOMMU backed device being
> removed, leaving a non-IOMMU backed device in the vfio_iommu) all
> sounds pretty complicated, especially as the vfio-iommu-type1 backend
> becomes stretched to be more and more fragile.  Possibly it's still
> feasible though.  Thanks,

Requiring file backed memory is not a restriction in practice, because there is
no way to preserve MAP_ANON memory across exec and map it into the new qemu process.
That is what MADV_DOEXEC would have provided.  Without it, one cannot do live 
update with MAP_ANON memory.

For qemu, when allocating anonymous memory for guest memory regions, we would modify the 
allocation  functions to call  memfd_create + mmap(fd) instead of mmap(MAP_ANON).  The
implementation of memfd_create creates a /dev/shm file and unlinks it. Thus the memory is 
backed by a file, and the VFIO UNMAP/REMAP proposal works.

- Steve