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

[David Gibson]

On Fri, Jun 09, 2017 at 09:58:47AM +0800, Peter Xu wrote:
> On Thu, Jun 08, 2017 at 09:59:50PM +0300, Michael S. Tsirkin wrote:
> > 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.
> 
> Do you mean to modify IOMMUTLBEntry.addr_mask into length?
> 
> Again, I am not sure this is good... At least we need to get ack from
> David since spapr should be the initial user of it, and possibly also
> Alex since vfio should be assuming that (IIUC both in QEMU and kernel)
> addr_mask is page masks rather than arbirary length.

So, I don't see that using size instead of mask would be a particular
problem for spapr.  However, I also don't see any advantage to
switching.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

signature.asc
Description: PGP signature