On Fri, Dec 14, 2018 at 10:43:03AM +0800, Jason Wang wrote:

On 2018/12/13 下午10:31, Michael S. Tsirkin wrote:

quoted

quoted

Just to make sure I understand this. It looks to me we should:

- allow passing GIOVA->GPA through UAPI

- cache GIOVA->GPA somewhere but still use GIOVA->HVA in device IOTLB for
performance

Is this what you suggest?

Thanks

Not really. We already have GPA->HVA, so I suggested a flag to pass
GIOVA->GPA in the IOTLB.

This has advantages for security since a single table needs
then to be validated to ensure guest does not corrupt
QEMU memory.

I wonder how much we can gain through this. Currently, qemu IOMMU gives
GIOVA->GPA mapping, and qemu vhost code will translate GPA to HVA then pass
GIOVA->HVA to vhost. It looks no difference to me.

Thanks

The difference is in security not in performance.  Getting a bad HVA
corrupts QEMU memory and it might be guest controlled. Very risky.  If
translations to HVA are done in a single place through a single table
it's safer as there's a single risky place.

-- 
MST