Re: [Qemu-devel] [PATCH RFC 0/4] intel_iommu: Do sanity check of vfio-pci earlier

[Peter Xu]

On Mon, Aug 12, 2019 at 09:45:27AM +0200, Peter Xu wrote:
> This is a RFC series.
> 
> The VT-d code has some defects, one of them is that we cannot detect
> the misuse of vIOMMU and vfio-pci early enough.
> 
> For example, logically this is not allowed:
> 
>   -device intel-iommu,caching-mode=off \
>   -device vfio-pci,host=05:00.0
> 
> Because the caching mode is required to make vfio-pci devices
> functional.
> 
> Previously we did this sanity check in vtd_iommu_notify_flag_changed()
> as when the memory regions change their attributes.  However that's
> too late in most cases!  Because the memory region layouts will only
> change after IOMMU is enabled, and that's in most cases during the
> guest OS boots.  So when the configuration is wrong, we will only bail
> out during the guest boots rather than simply telling the user before
> QEMU starts.
> 
> The same problem happens on device hotplug, say, when we have this:
> 
>   -device intel-iommu,caching-mode=off
> 
> Then we do something like:
> 
>   (HMP) device_add vfio-pci,host=05:00.0,bus=pcie.1
> 
> If at that time the vIOMMU is enabled in the guest then the QEMU
> process will simply quit directly due to this hotplug event.  This is
> a bit insane...
> 
> This series tries to solve above two problems by introducing two
> sanity checks upon these places separately:
> 
>   - machine done
>   - hotplug device
> 
> This is a bit awkward but I hope this could be better than before.
> There is of course other solutions like hard-code the check into
> vfio-pci but I feel it even more unpretty.  I didn't think out any
> better way to do this, if there is please kindly shout out.
> 
> Please have a look to see whether this would be acceptable, thanks.

Any more comment on this?

Thanks,

-- 
Peter Xu