Re: [Qemu-devel] [PATCH v1 2/3] intel-iommu: extend VTD emulation to allow 57-bit IOVA address width.

[Yu Zhang <yu.c.zhang@linux.intel.com>]

On Tue, Nov 13, 2018 at 02:12:17PM +0800, Peter Xu wrote:
> On Tue, Nov 13, 2018 at 01:45:44PM +0800, Yu Zhang wrote:
> 
> [...]
> 
> > > > Since at it, another thing I thought about is making sure the IOMMU
> > > > capabilities will match between host and guest IOMMU, which I think
> > > > this series has ignorred so far.  E.g., when we're having assigned
> > > > devices in the guest and with 5-level IOVA, we should make sure the
> > > > host IOMMU supports 5-level as well before the guest starts since
> > > > otherwise the shadow page synchronization could potentially fail when
> > > > the requested IOVA address goes beyond 4-level.  One simple solution
> > > > is just to disable device assignment for now when we're with 57bits
> > > > vIOMMU but I'm not sure whether that's what you want, especially you
> > > > mentioned the DPDK case (who may use assigned devices).
> > > 
> > > Ok I totally forgot that we don't even support any kind of check like
> > > this before... So feel free to skip this comment if you want, or it
> > > would be even nicer if you want to fix it as a whole. :)
> > > 
> > 
> > Indeed. We have talked about this before. How about we focus on the 5-level
> > extension for now, and solve the check issue in the future? I still do not
> > have any clean solutions in mind. BTW, any suggestions on this issue? :)
> 
> I started to remember our discussions, sorry I should remember them
> earlier... :)
> 
> The only thing in my mind (I think I also suggested the same thing
> during that discussion, but I don't trust my memory any more...) is to
> use sysfs.  Say:
> 
>   1. Scan /sys/class/iommu/dmarN for all the host IOMMUs, read cap of
>      each IOMMU from /sys/class/iommu/dmar0/intel-iommu/cap,
> 
>   2. For each host iommu, scan /sys/class/iommu/dmarN/devices for all
>      the devices under each host IOMMU, then we can know which IOMMU
>      owns which device,
> 
>   3. For each assigned device to the guest, we lookup the previous
>      information to know the mgaw for each host device, raise error
>      and stop QEMU from booting if any of the host device has less
>      level supported than the guest vIOMMU (possibly some more checks
>      in vtd_iommu_notify_flag_changed)
> 
> (we still have some issue on vtd_iommu_notify_flag_changed since it's
>  only run until the first enablement of vIOMMU, so we'll only raise
>  the error during guest Linux boots with vIOMMU on. But that's another
>  issue)

Thanks for the explanation, Peter. You do have a better memory than I am.:)


> 
> Regards,
> 
> -- 
> Peter Xu
> 

B.R.
Yu