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

[Michael S. Tsirkin]

On Thu, Jun 08, 2017 at 02:11:50PM +0800, Peter Xu wrote:
> On Wed, Jun 07, 2017 at 04:07:20PM +0300, Michael S. Tsirkin wrote:
> > 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.
> 
> The problem is that when I was fixing the problem that vhost had with
> PT (a764040, "exec: abstract address_space_do_translate()"), I did
> broke the IOTLB translation a bit (it was using page masks). IMHO we
> need to fix it first for correctness (patch 1/2).
> 
> For patch 3, if we can have Jason's patch to allow dynamic
> iommu_platform switching, that'll be the best, then I can rewrite
> patch 3 with the switching logic rather than caching anything. But
> IMHO that can be separated from patch 1/2 if you like.
> 
> Or do you have better suggestion on how should we fix it?
> 
> Thanks,

Can we drop masks completely and replace with length? I think we
should do that instead of trying to fix masks.

> -- 
> Peter Xu