Re: [Qemu-devel] [PATCH v7 14/17] memory: add MemoryRegionIOMMUOps.replay() callback

[Jason Wang]

On 2017年03月30日 19:06, Liu, Yi L wrote:
> > -----Original Message-----
> > From: Liu, Yi L
> > Sent: Monday, March 27, 2017 5:22 PM
> > To: Peter Xu <address@hidden>
> > Cc: address@hidden; Lan, Tianyu <address@hidden>; Tian, Kevin
> > <address@hidden>; address@hidden; address@hidden;
> > address@hidden; address@hidden; David Gibson
> > <address@hidden>; address@hidden
> > Subject: RE: [Qemu-devel] [PATCH v7 14/17] memory: add
> > MemoryRegionIOMMUOps.replay() callback
> >
> > > -----Original Message-----
> > > From: Peter Xu [mailto:address@hidden
> > > Sent: Monday, March 27, 2017 5:12 PM
> > > To: Liu, Yi L <address@hidden>
> > > Cc: address@hidden; Lan, Tianyu <address@hidden>;
> > > Tian, Kevin <address@hidden>; address@hidden;
> > > address@hidden; address@hidden; address@hidden; David
> > > Gibson <address@hidden>; address@hidden
> > > Subject: Re: [Qemu-devel] [PATCH v7 14/17] memory: add
> > > MemoryRegionIOMMUOps.replay() callback
> > >
> > > On Mon, Mar 27, 2017 at 08:35:05AM +0000, Liu, Yi L wrote:
> > > > > -----Original Message-----
> > > > > From: Qemu-devel
> > > > > [mailto:address@hidden On
> > > > > Behalf Of Peter Xu
> > > > > Sent: Tuesday, February 7, 2017 4:28 PM
> > > > > To: address@hidden
> > > > > Cc: Lan, Tianyu <address@hidden>; Tian, Kevin
> > > > > <address@hidden>; address@hidden; address@hidden;
> > > > > address@hidden; address@hidden;
> > > > > address@hidden; address@hidden; David Gibson
> > > > > <address@hidden>
> > > > > Subject: [Qemu-devel] [PATCH v7 14/17] memory: add
> > > > > MemoryRegionIOMMUOps.replay() callback
> > > > >
> > > > > Originally we have one memory_region_iommu_replay() function,
> > > > > which is the default behavior to replay the translations of the
> > > > > whole IOMMU region. However, on some platform like x86, we may
> > > > > want our own
> > > replay logic for IOMMU regions.
> > > > > This patch add one more hook for IOMMUOps for the callback, and
> > > > > it'll override the default if set.
> > > > >
> > > > > Signed-off-by: Peter Xu <address@hidden>
> > > > > ---
> > > > >   include/exec/memory.h | 2 ++
> > > > >   memory.c              | 6 ++++++
> > > > >   2 files changed, 8 insertions(+)
> > > > >
> > > > > diff --git a/include/exec/memory.h b/include/exec/memory.h index
> > > > > 0767888..30b2a74 100644
> > > > > --- a/include/exec/memory.h
> > > > > +++ b/include/exec/memory.h
> > > > > @@ -191,6 +191,8 @@ struct MemoryRegionIOMMUOps {
> > > > >       void (*notify_flag_changed)(MemoryRegion *iommu,
> > > > >                                   IOMMUNotifierFlag old_flags,
> > > > >                                   IOMMUNotifierFlag new_flags);
> > > > > +    /* Set this up to provide customized IOMMU replay function */
> > > > > +    void (*replay)(MemoryRegion *iommu, IOMMUNotifier *notifier);
> > > > >   };
> > > > >
> > > > >   typedef struct CoalescedMemoryRange CoalescedMemoryRange; diff
> > > > > --git a/memory.c b/memory.c index 7a4f2f9..9c253cc 100644
> > > > > --- a/memory.c
> > > > > +++ b/memory.c
> > > > > @@ -1630,6 +1630,12 @@ void
> > > > > memory_region_iommu_replay(MemoryRegion
> > > > > *mr, IOMMUNotifier *n,
> > > > >       hwaddr addr, granularity;
> > > > >       IOMMUTLBEntry iotlb;
> > > > > +    /* If the IOMMU has its own replay callback, override */
> > > > > +    if (mr->iommu_ops->replay) {
> > > > > +        mr->iommu_ops->replay(mr, n);
> > > > > +        return;
> > > > > +    }
> > > > Hi Alex, Peter,
> > > >
> > > > Will all the other vendors(e.g. PPC, s390, ARM) add their own replay
> > > > callback as well? I guess it depends on whether the original replay
> > > > algorithm work well for them? Do you have such knowledge?
> > > I guess so. At least for VT-d we had this callback since the default
> > > replay mechanism did not work well on x86 due to its extremely large
> > > memory region size. Thanks,
> > thx. that would make sense.
> Peter,
>
> Just come to mind that there may be a corner case here.
>
> Intel VT-d actually has a "pt" mode which allows device use physical address
> even when VT-d is enabled. In kernel, there is a iommu_identity_mapping.
> If a device is in this map, then it would use "pt" mode. So that IOMMU driver
> would not build second-level page table for it.

Yes, but qemu does not support ECAP_PT now, so guest will still have a 
page table in this case.

> Back to the virtual IOVA implementation, if an assigned device is in the
> iommu_identity_mapping(e.g. VGA controller), it uses GPA directly to do DMA.
> So it demands a GPA->HPA mapping in host. However, the iommu->ops.replay
> is not able to build it when guest SL page table is empty.
>
> So I think building an entire guest PA->HPA mapping before guest kernel boot
> would be recommended. Any thoughts?

We plan to add PT in 2.10, a possible rough idea is disabled iommu dmar 
region and use another region without iommu_ops. Then 
vfio_listener_region_add() will just do the correct mappings.

Thanks

> Regards,
> Yi L