Re: [Qemu-devel] [PATCH v8 01/13] vfio: KABI for migration interface

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 30 Aug 2019 08:06:32 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Tian, Kevin
> > Sent: Friday, August 30, 2019 3:26 PM
> >   
> [...]
> > > How does QEMU handle the fact that IOVAs are potentially dynamic while
> > > performing the live portion of a migration?  For example, each time a
> > > guest driver calls dma_map_page() or dma_unmap_page(), a
> > > MemoryRegionSection pops in or out of the AddressSpace for the device
> > > (I'm assuming a vIOMMU where the device AddressSpace is not
> > > system_memory).  I don't see any QEMU code that intercepts that change
> > > in the AddressSpace such that the IOVA dirty pfns could be recorded and
> > > translated to GFNs.  The vendor driver can't track these beyond getting
> > > an unmap notification since it only knows the IOVA pfns, which can be
> > > re-used with different GFN backing.  Once the DMA mapping is torn down,
> > > it seems those dirty pfns are lost in the ether.  If this works in QEMU,
> > > please help me find the code that handles it.  
> > 
> > I'm curious about this part too. Interestingly, I didn't find any log_sync
> > callback registered by emulated devices in Qemu. Looks dirty pages
> > by emulated DMAs are recorded in some implicit way. But KVM always
> > reports dirty page in GFN instead of IOVA, regardless of the presence of
> > vIOMMU. If Qemu also tracks dirty pages in GFN for emulated DMAs
> >  (translation can be done when DMA happens), then we don't need
> > worry about transient mapping from IOVA to GFN. Along this way we
> > also want GFN-based dirty bitmap being reported through VFIO,
> > similar to what KVM does. For vendor drivers, it needs to translate
> > from IOVA to HVA to GFN when tracking DMA activities on VFIO
> > devices. IOVA->HVA is provided by VFIO. for HVA->GFN, it can be
> > provided by KVM but I'm not sure whether it's exposed now.
> >   
> 
> HVA->GFN can be done through hva_to_gfn_memslot in kvm_host.h.

I thought it was bad enough that we have vendor drivers that depend on
KVM, but designing a vfio interface that only supports a KVM interface
is more undesirable.  I also note without comment that gfn_to_memslot()
is a GPL symbol.  Thanks,

Alex

> Above flow works for software-tracked dirty mechanism, e.g. in
> KVMGT, where GFN-based 'dirty' is marked when a guest page is 
> mapped into device mmu. IOVA->HPA->GFN translation is done 
> at that time, thus immune from further IOVA->GFN changes.
> 
> When hardware IOMMU supports D-bit in 2nd level translation (e.g.
> VT-d rev3.0), there are two scenarios:
> 
> 1) nested translation: guest manages 1st-level translation (IOVA->GPA)
> and host manages 2nd-level translation (GPA->HPA). The 2nd-level
> is not affected by guest mapping operations. So it's OK for IOMMU
> driver to retrieve GFN-based dirty pages by directly scanning the 2nd-
> level structure, upon request from user space. 
> 
> 2) shadowed translation (IOVA->HPA) in 2nd level: in such case the dirty
> information is tied to IOVA. the IOMMU driver is expected to maintain
> an internal dirty bitmap. Upon any change of IOVA->GPA notification
> from VFIO, the IOMMU driver should flush dirty status of affected 2nd-level
> entries to the internal GFN-based bitmap. At this time, again IOVA->HVA
> ->GPA translation required for GFN-based recording. When userspace   
> queries dirty bitmap, the IOMMU driver needs to flush latest 2nd-level 
> dirty status to internal bitmap, which is then copied to user space.
> 
> Given the trickiness of 2), we aim to enable 1) on intel-iommu driver.
> 
> Thanks
> Kevin