Re: [Qemu-devel] [PATCH v1 2/2] intel-iommu: Extend address width to 48 bits

[Liu, Yi L]

On Thu, Nov 30, 2017 at 05:11:55PM +0800, Peter Xu wrote:
> On Thu, Nov 30, 2017 at 01:22:38PM +0800, Liu, Yi L wrote:
> > On Tue, Nov 14, 2017 at 06:13:50PM -0500, address@hidden wrote:
> > > From: Prasad Singamsetty <address@hidden>
> > > 
> > > The current implementation of Intel IOMMU code only supports 39 bits
> > > iova address width. This patch provides a new parameter (x-aw-bits)
> > > for intel-iommu to extend its address width to 48 bits but keeping the
> > > default the same (39 bits). The reason for not changing the default
> > > is to avoid potential compatibility problems with live migration of
> > > intel-iommu enabled QEMU guest. The only valid values for 'x-aw-bits'
> > > parameter are 39 and 48.
> > > 
> > > After enabling larger address width (48), we should be able to map
> > > larger iova addresses in the guest. For example, a QEMU guest that
> > > is configured with large memory ( >=1TB ). To check whether 48 bits
> > 
> > I didn't quite get your point here. Address width limits the iova range,
> > but it doesn't limit the guest memory range. e.g. you can use 39 bit iova
> > address to access a guest physical address larger than (2^39 - 1) as long
> > as the guest 2nd level page table is well programmed. Only one exception,
> > if you requires a continuous iova range(e.g. 2^39), it would be an issue.
> > Not sure if this is the major motivation of your patch? However, I'm not
> > against extend the address width to be 48 bits. Just need to make it clear
> > here.
> 
> One thing I can think of is the identity mapping. Say, when iommu=pt
> is set in guest, meanwhile when PT capability is not supported from
> hardware (here I mean, the emulated hardware, of course), guest kernel
> will create one identity mapping to emulate the PT mode.

True.
 
> Current linux kernel's identity mapping should be a static 48 bits
> mapping (it must cover the whole memory range of guest), so if we

I suppose guest memory range depends on the AW reported by CPUID? Not sure
if it is constantly 48 bits.

> provide a 39 bits vIOMMU to the guest, AFAIU we'll fail at device
> attaching to that identity domain of every single device that is
> protected by that 39 bits vIOMMU (kernel will find that domain gaw is
> bigger than vIOMMU supported gaw of that device).

Yeah, this is a good argue. As it is 1:1 mapping, the translated address
is limited all the same.

> I do see no good fix for that, except boost the supported gaw to
> bigger ones.

How about defaultly expose cap.PT bit? In that case, there will no 1:1
mapping in guest side. Translation is skipped. So the IOMMU AW won't
limit the addressing.

Regards,
Yi L

> 
> Thanks,
> 
> -- 
> Peter Xu
>