Re: [Qemu-devel] [PATCH v3 1/2] intel-iommu: differentiate host address width from IOVA address width.

[Eduardo Habkost]

On Wed, Dec 26, 2018 at 01:30:00PM +0800, Yu Zhang wrote:
> On Tue, Dec 25, 2018 at 11:56:19AM -0500, Michael S. Tsirkin wrote:
> > On Sat, Dec 22, 2018 at 09:11:26AM +0800, Yu Zhang wrote:
> > > On Fri, Dec 21, 2018 at 02:02:28PM -0500, Michael S. Tsirkin wrote:
> > > > On Sat, Dec 22, 2018 at 01:37:58AM +0800, Yu Zhang wrote:
> > > > > On Fri, Dec 21, 2018 at 12:04:49PM -0500, Michael S. Tsirkin wrote:
> > > > > > On Sat, Dec 22, 2018 at 12:09:44AM +0800, Yu Zhang wrote:
> > > > > > > Well, my understanding of the vt-d spec is that the address 
> > > > > > > limitation in
> > > > > > > DMAR are referring to the same concept of CPUID.MAXPHYSADDR. I do 
> > > > > > > not think
> > > > > > > there's any different in the native scenario. :)
> > > > > > 
> > > > > > I think native machines exist on which the two values are different.
> > > > > > Is that true?
> > > > > 
> > > > > I think the answer is not. My understanding is that HAW(host address 
> > > > > wdith) is
> > > > > the maximum physical address width a CPU can detects(by 
> > > > > cpuid.0x80000008).
> > > > > 
> > > > > I agree there are some addresses the CPU does not touch, but they are 
> > > > > still in
> > > > > the physical address space, and there's only one physical address 
> > > > > space...
> > > > > 
> > > > > B.R.
> > > > > Yu
> > > > 
> > > > Ouch I thought we are talking about the virtual address size.
> > > > I think I did have a box where VTD's virtual address size was
> > > > smaller than CPU's.
> > > > For physical one - we just need to make it as big as max supported
> > > > memory right?
> > > 
> > > Well, my understanding of the physical one is the maximum physical address
> > > width. Sorry, this explain seems nonsense... I mean, it's not just about
> > > the max supported memory, but also covers MMIO. It shall be detectable
> > > from cpuid, or ACPI's DMAR table, instead of calculated by the max memory
> > > size. One common usage of this value is to tell the paging structure 
> > > entries(
> > > CPU's or IOMMU's) which bits shall be reserved. There are also some 
> > > registers
> > > e.g. apic base reg etc, whose contents are physical addresses, therefore 
> > > also
> > > need to follow the similar requirement for the reserved bits.
> > > 
> > > So I think the correct direction might be to define this property in the
> > > machine status level, instead of the CPU level. Is this reasonable to you?
> > 
> > At that level yes. But isn't this already specified by "pci-hole64-end"?
> 
> But this value is set by guest firmware? Will PCI hotplug change this address?
> 
> @Eduardo, do you have any plan to calculate the phys-bits by "pci-hole64-end"?
> Or introduce another property, say "max-phys-bits" in machine status?

I agree it may make sense to make the machine code control
phys-bits instead of the CPU object.  A machine property sounds
like the simplest solution.

But I don't think we can have a meaningful discussion about
implementation if we don't agree about the command-line
interface.  We must decide what will happen to the CPU and iommu
physical address width in cases like:

  $QEMU -device intel-iommu
  $QEMU -cpu ...,phys-bits=50 -device intel-iommu
  $QEMU -cpu ...,host-phys-bits=on -device intel-iommu
  $QEMU -machine phys-bits=50 -device intel-iommu
  $QEMU -machine phys-bits=50 -cpu ...,phys-bits=48 -device intel-iommu

-- 
Eduardo