Re: [Xen-devel] Xen virtual IOMMU high level design doc V3

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Monday, November 21, 2016 9:41 PM
> 
> On 17/11/16 15:36, Lan Tianyu wrote:
> > 3.2 l2 translation
> > 1) For virtual PCI device
> > Xen dummy xen-vIOMMU in Qemu translates IOVA to target GPA via new
> > hypercall when DMA operation happens.
> >
> > When guest triggers a invalidation operation, there maybe in-fly DMA
> > request for virtual device has been translated by vIOMMU and return back
> > Qemu. Before vIOMMU tells invalidation completed, it's necessary to make
> > sure in-fly DMA operation is completed.
> >
> > When IOMMU driver invalidates IOTLB, it also will wait until the
> > invalidation completion. We may use this to drain in-fly DMA operation
> > for virtual device.
> >
> > Guest triggers invalidation operation and trip into vIOMMU in
> > hypervisor to flush cache data. After this, it should go to Qemu to
> > drain in-fly DMA translation.
> >
> > To do that, dummy vIOMMU in Qemu registers the same MMIO region as
> > vIOMMU's and emulation part of invalidation operation in Xen hypervisor
> > returns X86EMUL_UNHANDLEABLE after flush cache. MMIO emulation part is
> > supposed to send event to Qemu and dummy vIOMMU get a chance to starts a
> > thread to drain in-fly DMA and return emulation done.
> >
> > Guest polls IVT(invalidate IOTLB) bit in the IOTLB invalidate register
> > until it's cleared after triggering invalidation. Dummy vIOMMU in Qemu
> > notifies hypervisor drain operation completed via hypercall, vIOMMU
> > clears IVT bit and guest finish invalidation operation.
> 
> Having the guest poll will be very inefficient.  If the invalidation
> does need to reach qemu, it will be a very long time until it
> completes.  Is there no interrupt based mechanism which can be used?
> That way the guest can either handle it asynchronous itself, or block
> waiting on an interrupt, both of which are better than having it just
> spinning.
> 

VT-d spec supports both poll and interrupt modes, and it's decided
by guest IOMMU driver. Not say that this design requires guest to
use poll mode. I guess Tianyu uses as an example flow.

Thanks
Kevin
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