Re: [Qemu-devel] [PATCH RFC v4 18/20] intel_iommu: enable vfio devices

[Alex Williamson]

On Wed, 25 Jan 2017 09:19:25 +0800
Jason Wang <address@hidden> wrote:

> On 2017年01月24日 03:40, Alex Williamson wrote:
> > On Mon, 23 Jan 2017 18:23:44 +0800
> > Jason Wang<address@hidden>  wrote:
> >  
> >> On 2017年01月23日 11:34, Peter Xu wrote:  
> >>> On Mon, Jan 23, 2017 at 09:55:39AM +0800, Jason Wang wrote:  
> >>>> On 2017年01月22日 17:04, Peter Xu wrote:  
> >>>>> On Sun, Jan 22, 2017 at 04:08:04PM +0800, Jason Wang wrote:
> >>>>>
> >>>>> [...]
> >>>>>     
> >>>>>>> +static void vtd_iotlb_page_invalidate_notify(IntelIOMMUState *s,
> >>>>>>> +                                           uint16_t domain_id, 
> >>>>>>> hwaddr addr,
> >>>>>>> +                                           uint8_t am)
> >>>>>>> +{
> >>>>>>> +    IntelIOMMUNotifierNode *node;
> >>>>>>> +    VTDContextEntry ce;
> >>>>>>> +    int ret;
> >>>>>>> +
> >>>>>>> +    QLIST_FOREACH(node, &(s->notifiers_list), next) {
> >>>>>>> +        VTDAddressSpace *vtd_as = node->vtd_as;
> >>>>>>> +        ret = vtd_dev_to_context_entry(s, pci_bus_num(vtd_as->bus),
> >>>>>>> +                                       vtd_as->devfn, &ce);
> >>>>>>> +        if (!ret && domain_id == VTD_CONTEXT_ENTRY_DID(ce.hi)) {
> >>>>>>> +            vtd_page_walk(&ce, addr, addr + (1 << am) * 
> >>>>>>> VTD_PAGE_SIZE,
> >>>>>>> +                          vtd_page_invalidate_notify_hook,
> >>>>>>> +                          (void *)&vtd_as->iommu, true);  
> >>>>>> Why not simply trigger the notifier here? (or is this vfio required?)  
> >>>>> Because we may only want to notify part of the region - we are with
> >>>>> mask here, but not exact size.
> >>>>>
> >>>>> Consider this: guest (with caching mode) maps 12K memory (4K*3 pages),
> >>>>> the mask will be extended to 16K in the guest. In that case, we need
> >>>>> to explicitly go over the page entry to know that the 4th page should
> >>>>> not be notified.  
> >>>> I see. Then it was required by vfio only, I think we can add a fast path 
> >>>> for
> >>>> !CM in this case by triggering the notifier directly.  
> >>> I noted this down (to be further investigated in my todo), but I don't
> >>> know whether this can work, due to the fact that I think it is still
> >>> legal that guest merge more than one PSIs into one. For example, I
> >>> don't know whether below is legal:
> >>>
> >>> - guest invalidate page (0, 4k)
> >>> - guest map new page (4k, 8k)
> >>> - guest send single PSI of (0, 8k)
> >>>
> >>> In that case, it contains both map/unmap, and looks like it didn't
> >>> disobay the spec as well?  
> >> Not sure I get your meaning, you mean just send single PSI instead of two?
> >>  
> >>>     
> >>>> Another possible issue is, consider (with CM) a 16K contiguous iova with 
> >>>> the
> >>>> last page has already been mapped. In this case, if we want to map first
> >>>> three pages, when handling IOTLB invalidation, am would be 16K, then the
> >>>> last page will be mapped twice. Can this lead some issue?  
> >>> I don't know whether guest has special handling of this kind of
> >>> request.  
> >> This seems quite usual I think? E.g iommu_flush_iotlb_psi() did:
> >>
> >> static void iommu_flush_iotlb_psi(struct intel_iommu *iommu,
> >>                     struct dmar_domain *domain,
> >>                     unsigned long pfn, unsigned int pages,
> >>                     int ih, int map)
> >> {
> >>       unsigned int mask = ilog2(__roundup_pow_of_two(pages));
> >>       uint64_t addr = (uint64_t)pfn << VTD_PAGE_SHIFT;
> >>       u16 did = domain->iommu_did[iommu->seq_id];
> >> ...
> >>
> >>  
> >>> Besides, imho to completely solve this problem, we still need that
> >>> per-domain tree. Considering that currently the tree is inside vfio, I
> >>> see this not a big issue as well.  
> >> Another issue I found is: with this series, VFIO_IOMMU_MAP_DMA seems
> >> become guest trigger-able. And since VFIO allocate its own structure to
> >> record dma mapping, this seems open a window for evil guest to exhaust
> >> host memory which is even worse.  
> > You're thinking of pci-assign, vfio does page accounting such that a
> > user can only lock pages up to their locked memory limit.  Exposing the
> > mapping ioctl within the guest is not a different problem from exposing
> > the ioctl to the host user from a vfio perspective.  Thanks,
> >
> > Alex
> >  
> 
> Yes, but what if an evil guest that maps all iovas to the same gpa?

Doesn't matter, we'd account that gpa each time it's mapped, so
effectively the locked memory limit is equal to the iova size the user
can map.  Thanks,

Alex