On 01/28/2016 12:19 AM, Alex Williamson wrote:
> On Wed, 2016-01-27 at 13:43 +0800, Jike Song wrote:
{snip}
>> Had a look at eventfd, I would say yes, technically we are able to
>> achieve the goal: introduce a fd, with fop->{read|write} defined in KVM,
>> call into vgpu device-model, also an iodev registered for a MMIO GPA
>> range to invoke the fop->{read|write}. I just didn't understand why
>> userspace can't register an iodev via API directly.
>
> Please elaborate on how it would work via iodev.
>
QEMU forwards BAR0 write to the bus driver, in the bus driver, if
found that MEM bit is enabled, register an iodev to KVM: with an
ops:
const struct kvm_io_device_ops trap_mmio_ops = {
.read = kvmgt_guest_mmio_read,
.write = kvmgt_guest_mmio_write,
};
I may not be able to illustrated it clearly with descriptions but this
should not be a problem, thanks to your explanation, I can understand
and adopt it for KVMGT.
>> Besides, this doesn't necessarily require another thread, right?
>> I guess it can be within the VCPU thread?
>
> I would think so too, the vcpu is blocked on the MMIO access, we should
> be able to service it in that context. I hope.
>
Thanks for confirmation.
>> And this brought another question: except the vfio bus drvier and
>> iommu backend (and the page_track ulitiy used for guest memory
>> write-protection),
>> is it KVMGT allowed to call into kvm.ko (or modify)? Though we are
>> becoming less and less willing to do that with VFIO, it's still better
>> to know that before going wrong.
>
> kvm and vfio are separate modules, for the most part, they know nothing
> about each other and have no hard dependencies between them. We do have
> various accelerations we can use to avoid paths through userspace, but
> these are all via APIs that are agnostic of the party on the other end.
> For example, vfio signals interrups through eventfds and has no concept
> of whether that eventfd terminates in userspace or into an irqfd in KVM.
> vfio supports direct access to device MMIO regions via mmaps, but vfio
> has no idea if that mmap gets directly mapped into a VM address space.
> Even with posted interrupts, we've introduced an irq bypass manager
> allowing interrupt producers and consumers to register independently to
> form a connection without directly knowing anything about the other
> module. That sort or proper software layering needs to continue. It
> would be wrong for a vfio bus driver to assume KVM is the user and
> directly call into KVM interfaces. Thanks,
>
I understand and agree with your point, it's bad if the bus driver
assume KVM is the user and/or call into KVM interfaces.
However, the vgpu device-model, in intel case also a part of i915 driver,
will always need to call some hypervisor-specific interfaces.
For example, when a guest gfx driver submit GPU commands, the device-model
may want to scan it for security or whatever-else purpose:
- get a GPA (from GPU page tables)
- want to read 16 bytes from that GPA
- call hypervisor-specific read_gpa() method
- for Xen, the GPA belongs to a foreign domain, it must find
a way to map & read it - beyond our scope here;
- for KVM, the GPA can converted to HVA, copy_from_user (if
called from vcpu thread) or access_remote_vm (if called from
other threads);
Please note that this is not from the vfio bus driver, but from the vgpu
device-model; also this is not DMA addr from GPU talbes, but real GPA.
> Alex
>
--
Thanks,
Jike
