Re: [PATCH 0/2] KVM: MMU: support VMAs that got remap_pfn_range-ed

[Neo Jia]

On Tue, Jul 05, 2016 at 12:02:42PM +0800, Xiao Guangrong wrote:
> 
> 
> On 07/05/2016 09:35 AM, Neo Jia wrote:
> >On Tue, Jul 05, 2016 at 09:19:40AM +0800, Xiao Guangrong wrote:
> >>
> >>
> >>On 07/04/2016 11:33 PM, Neo Jia wrote:
> >>
> >>>>>
> >>>>>Sorry, I think I misread the "allocation" as "mapping". We only delay the
> >>>>>cpu mapping, not the allocation.
> >>>>
> >>>>So how to understand your statement:
> >>>>"at that moment nobody has any knowledge about how the physical mmio gets virtualized"
> >>>>
> >>>>The resource, physical MMIO region, has been allocated, why we do not know the physical
> >>>>address mapped to the VM?
> >>>>
> >>>
> >>>>From a device driver point of view, the physical mmio region never gets allocated until
> >>>the corresponding resource is requested by clients and granted by the mediated device driver.
> >>
> >>Hmm... but you told me that you did not delay the allocation. :(
> >
> >Hi Guangrong,
> >
> >The allocation here is the allocation of device resource, and the only way to
> >access that kind of device resource is via a mmio region of some pages there.
> >
> >For example, if VM needs resource A, and the only way to access resource A is
> >via some kind of device memory at mmio address X.
> >
> >So, we never defer the allocation request during runtime, we just setup the
> >CPU mapping later when it actually gets accessed.
> >
> >>
> >>So it returns to my original question: why not allocate the physical mmio region in mmap()?
> >>
> >
> >Without running anything inside the VM, how do you know how the hw resource gets
> >allocated, therefore no knowledge of the use of mmio region.
> 
> The allocation and mapping can be two independent processes:
> - the first process is just allocation. The MMIO region is allocated from physical
>   hardware and this region is mapped into _QEMU's_ arbitrary virtual address by mmap().
>   At this time, VM can not actually use this resource.
> 
> - the second process is mapping. When VM enable this region, e.g, it enables the
>   PCI BAR, then QEMU maps its virtual address returned by mmap() to VM's physical
>   memory. After that, VM can access this region.
> 
> The second process is completed handled in userspace, that means, the mediated
> device driver needn't care how the resource is mapped into VM.

In your example, you are still picturing it as VFIO direct assign, but the solution we are 
talking here is mediated passthru via VFIO framework to virtualize DMA devices without SR-IOV.

(Just for completeness, if you really want to use a device in above example as
VFIO passthru, the second step is not completely handled in userspace, it is actually the guest
driver who will allocate and setup the proper hw resource which will later ready
for you to access via some mmio pages.)

> 
> This is how QEMU/VFIO currently works, could you please tell me the special points
> of your solution comparing with current QEMU/VFIO and why current model can not fit
> your requirement? So that we can better understand your scenario?

The scenario I am describing here is mediated passthru case, but what you are
describing here (more or less) is VFIO direct assigned case. It is different in several
areas, but major difference related to this topic here is:

1) In VFIO direct assigned case, the device (and its resource) is completely owned by the VM
therefore its mmio region can be mapped directly into the VM during the VFIO mmap() call as
there is no resource sharing among VMs and there is no mediated device driver on
the host to manage such resource, so it is completely owned by the guest.

2) In mediated passthru case, multiple VMs are sharing the same physical device, so how
the HW resource gets allocated is completely decided by the guest and host device driver of 
the virtualized DMA device, here is the GPU, same as the MMIO pages used to access those Hw resource.

Thanks,
Neo