Re: [Qemu-devel] [PATCH 2/3] exec: simplify address_space_get_iotlb_entry

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jun 07, 2017 at 11:44:43AM +0800, Peter Xu wrote:
> On Wed, Jun 07, 2017 at 09:47:05AM +1000, David Gibson wrote:
> > On Tue, Jun 06, 2017 at 04:34:30PM +0200, Paolo Bonzini wrote:
> > > 
> > > 
> > > On 05/06/2017 05:07, Peter Xu wrote:
> > > > I don't sure whether it'll be a good interface for IOTLB. AFAIU at
> > > > least for VT-d, the IOMMU translation is page aligned which is defined
> > > > by spec, so it makes sense that (again at least for VT-d) here we'd
> > > > better just use page_mask/addr_mask.
> > > > 
> > > > That's also how I know about IOMMU in general - I assume it do the
> > > > translations always with page masks (never arbitary length), though
> > > > page size can differ from platfrom to platform, that's why here the
> > > > IOTLB interface used addr_mask, then it works for all platforms. I
> > > > don't know whether I'm 100% correct here though.
> > > > 
> > > > Maybe David/Paolo/... would comment as well?
> > > 
> > > I would ask David.  There are PowerPC MMUs that allow fast lookup of
> > > arbitrarily-sized windows (not necessarily power of two),
> > 
> > Uh.. I'm not sure what you mean here.  You might be thinking of the
> > BATs which really old (32-bit) PowerPC MMUs had - those allow
> > arbitrary large block translations, but they do have to be a power of
> > two.
> > 
> > > so maybe the
> > > IOMMUs can do the same.
> > 
> > The only Power IOMMU I know about uses a fixed, power-of-two page size
> > per DMA window.
> 
> If so, I would still be inclined to keep using masks for QEMU IOTLB.
> Then, my first two patches should still stand.
> 
> I am just afraid that not using masks will diverge the emulation from
> real hardware and brings trouble one day.
> 
> For vhost IOTLB interface, it does not need to be strictly aligned to
> QEMU IOMMU IOTLB definition, and that's how it's working now (current
> vhost iotlb allows arbitary length, and I think it's good). So imho we
> don't really need to worry about the performance - after all, we can
> do everything customized for vhost, just like what patch 3 did (yeah,
> it can be better...).
> 
> Thanks,

Pre-faults is also something that does not happen on real hardware.
And it's about security so a bigger issue.

If I had to choose between that and using non-power-of-2 in
the API, I'd go for non-power-of-2. Let backends that can only
support power of 2 split it up to multiple transactions.



> -- 
> Peter Xu