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

[Neo Jia]

On Tue, Jul 05, 2016 at 05:02:46PM +0800, Xiao Guangrong wrote:
> 
> >
> >It is physically contiguous but it is done during the runtime, physically contiguous doesn't mean
> >static partition at boot time. And only during runtime, the proper HW resource will be requested therefore
> >the right portion of MMIO region will be granted by the mediated device driver on the host.
> 
> Okay. This is your implantation design rather than the hardware limitation, right?

I don't think it matters here. We are talking about framework so it should
provide the flexibility for different driver vendor.

> 
> For example, if the instance require 512M memory (the size can be specified by QEMU
> command line), it can tell its requirement to the mediated device driver via create()
> interface, then the driver can allocate then memory for this instance before it is running.

BAR != your device memory

We don't set the BAR size via QEMU command line, BAR size is extracted by QEMU
from config space provided by vendor driver.

> 
> Theoretically, the hardware is able to do memory management as this style, but for some
> reasons you choose allocating memory in the runtime. right? If my understanding is right,
> could you please tell us what benefit you want to get from this running-allocation style?

Your understanding is incorrect.

> 
> >
> >Then the req_size and pgoff will both come from the mediated device driver based on his internal book
> >keeping of the hw resource allocation, which is only available during runtime. And such book keeping
> >can be built part of para-virtualization scheme between guest and host device driver.
> >
> 
> I am talking the parameters you passed to validate_map_request(). req_size is calculated like this:
> 
> +       offset   = virtaddr - vma->vm_start;
> +       phyaddr  = (vma->vm_pgoff << PAGE_SHIFT) + offset;
> +       pgoff    = phyaddr >> PAGE_SHIFT;
> 
> All these info is from vma which is available in mmmap().
> 
> pgoff is got from:
> +       pg_prot  = vma->vm_page_prot;
> that is also available in mmap().

This is kept there in case the validate_map_request() is not provided by vendor
driver then by default assume 1:1 mapping. So if validate_map_request() is not
provided, fault handler should not fail.

> 
> >None of such information is available at VFIO mmap() time. For example, several VMs
> >are sharing the same physical device to provide mediated access. All VMs will
> >call the VFIO mmap() on their virtual BAR as part of QEMU vfio/pci initialization
> >process, at that moment, we definitely can't mmap the entire physical MMIO
> >into both VM blindly for obvious reason.
> >
> 
> mmap() carries @length information, so you only need to allocate the specified size
> (corresponding to @length) of memory for them.

Again, you still look at this as a static partition at QEMU configuration time
where the guest mmio will be mapped as a whole at some offset of the physical
mmio region. (You still can do that like I said above by not providing
validate_map_request() in your vendor driver.)

But this is not the framework we are defining here.

The framework we have here is to provide the driver vendor flexibility to decide
the guest mmio and physical mmio mapping on page basis, and such information is
available during runtime.

How such information gets communicated between guest and host driver is up to
driver vendor.

Thanks,
Neo