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

[Alex Williamson]

On Tue, 17 Dec 2019 00:25:02 -0500
Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:

> On Tue, Dec 17, 2019 at 01:17:29PM +0800, Tian, Kevin wrote:
> > > From: Tian, Kevin
> > > Sent: Tuesday, December 17, 2019 10:29 AM
> > >   
> > > > 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.  
> > 
> > One correction. vfio_pin_pages doesn't involve iommu invalidation. I should
> > just mean that pinning the page is not necessary. We just need a kvm-like
> > interface based on hva to access.
> >  
> And can we propose to differentiate read and write when calling vfio_pin_pages, e.g.
> vfio_pin_pages_read, vfio_pin_pages_write? Otherwise, calling to
> vfio_pin_pages will unnecessarily cause read pages to be dirty and
> sometimes reading guest pages is a way for device model to track dirty
> pages.

Yes, I've discussed this with Kirti, when devices add more fine grained
dirty tracking we'll probably need to extend the mdev pinned pages
interface to allow vendor drivers to indicate a pinning is intended to
be used as read-only and perhaps also a way to unpin a page that was
pinned as read-write as clean, if the device did not write to it.  So
perhaps vfio_pin_pages_for_read() and vfio_unpin_pages_clean().  Thanks,

Alex