Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

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

On Tue, 17 Dec 2019 02:28:33 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Paolo Bonzini
> > Sent: Monday, December 16, 2019 6:08 PM
> > 
> > [Alex and Kevin: there are doubts below regarding dirty page tracking
> > from VFIO and mdev devices, which perhaps you can help with]
> > 
> > On 15/12/19 18:21, Peter Xu wrote:  
> > >                 init_rmode_tss
> > >                     vmx_set_tss_addr
> > >                         kvm_vm_ioctl_set_tss_addr [*]
> > >                 init_rmode_identity_map
> > >                     vmx_create_vcpu [*]  
> > 
> > These don't matter because their content is not visible to userspace
> > (the backing storage is mmap-ed by __x86_set_memory_region).  In fact, d
> >   
> > >                 vmx_write_pml_buffer
> > >                     kvm_arch_write_log_dirty [&]
> > >                 kvm_write_guest
> > >                     kvm_hv_setup_tsc_page
> > >                         kvm_guest_time_update [&]
> > >                     nested_flush_cached_shadow_vmcs12 [&]
> > >                     kvm_write_wall_clock [&]
> > >                     kvm_pv_clock_pairing [&]
> > >                     kvmgt_rw_gpa [?]  
> > 
> > This then expands (partially) to
> > 
> > intel_gvt_hypervisor_write_gpa
> >     emulate_csb_update
> >         emulate_execlist_ctx_schedule_out
> >             complete_execlist_workload
> >                 complete_current_workload
> >                      workload_thread
> >         emulate_execlist_ctx_schedule_in
> >             prepare_execlist_workload
> >                 prepare_workload
> >                     dispatch_workload
> >                         workload_thread
> > 
> > So KVMGT is always writing to GPAs instead of IOVAs and basically
> > bypassing a guest IOMMU.  So here it would be better if kvmgt was
> > changed not use kvm_write_guest (also because I'd probably have nacked
> > that if I had known :)).  
> 
> I agree. 
> 
> > 
> > As far as I know, there is some work on live migration with both VFIO
> > and mdev, and that probably includes some dirty page tracking API.
> > kvmgt could switch to that API, or there could be VFIO APIs similar to
> > kvm_write_guest but taking IOVAs instead of GPAs.  Advantage: this would
> > fix the GPA/IOVA confusion.  Disadvantage: userspace would lose the
> > tracking of writes from mdev devices.  Kevin, are these writes used in
> > any way?  Do the calls to intel_gvt_hypervisor_write_gpa covers all
> > writes from kvmgt vGPUs, or can the hardware write to memory as well
> > (which would be my guess if I didn't know anything about kvmgt, which I
> > pretty much don't)?  
> 
> intel_gvt_hypervisor_write_gpa covers all writes due to software mediation.
> 
> for hardware updates, it needs be mapped in IOMMU through vfio_pin_pages 
> before any DMA happens. The ongoing dirty tracking effort in VFIO will take
> every pinned page through that API as dirtied.
> 
> However, currently VFIO doesn't implement any vfio_read/write_guest
> interface yet. and it doesn't make sense to use vfio_pin_pages for software
> dirtied pages, as pin is unnecessary and heavy involving iommu invalidation.
> 
> Alex, if you are OK we'll work on such interface and move kvmgt to use it.
> After it's accepted, we can also mark pages dirty through this new interface
> in Kirti's dirty page tracking series.

I'm not sure what you're asking for, is it an interface for the host
CPU to read/write the memory backing of a mapped IOVA range without
pinning pages?  That seems like something like that would make sense for
an emulation model where a page does not need to be pinned for physical
DMA.  If you're asking more for an interface that understands the
userspace driver is a VM (ie. implied using a _guest postfix on the
function name) and knows about GPA mappings beyond the windows directly
mapped for device access, I'd not look fondly on such a request.
Thanks,

Alex