Re: [RFC PATCH 4/9] vfio-pci: register default dynamic-trap-bar-info region

Yan Zhao
2019-12-12T03:19:53Z

On Thu, Dec 12, 2019 at 11:07:42AM +0800, Alex Williamson wrote:
> On Wed, 11 Dec 2019 21:02:40 -0500
> Yan Zhao <yan.y.zhao@intel.com> wrote:
>
> > On Thu, Dec 12, 2019 at 02:56:55AM +0800, Alex Williamson wrote:
> > > On Wed, 11 Dec 2019 01:25:55 -0500
> > > Yan Zhao <yan.y.zhao@intel.com> wrote:
> > >
> > > > On Wed, Dec 11, 2019 at 12:38:05AM +0800, Alex Williamson wrote:
> > > > > On Tue, 10 Dec 2019 02:44:44 -0500
> > > > > Yan Zhao <yan.y.zhao@intel.com> wrote:
> > > > >
> > > > > > On Tue, Dec 10, 2019 at 05:16:08AM +0800, Alex Williamson wrote:
> > > > > > > On Mon, 9 Dec 2019 01:22:12 -0500
> > > > > > > Yan Zhao <yan.y.zhao@intel.com> wrote:
> > > > > > >
> > > > > > > > On Fri, Dec 06, 2019 at 11:20:38PM +0800, Alex Williamson wrote:
> > > > > > > > > On Fri, 6 Dec 2019 01:04:07 -0500
> > > > > > > > > Yan Zhao <yan.y.zhao@intel.com> wrote:
> > > > > > > > >
> > > > > > > > > > On Fri, Dec 06, 2019 at 07:55:30AM +0800, Alex Williamson wrote:
> > > > > > > > > > > On Wed, 4 Dec 2019 22:26:50 -0500
> > > > > > > > > > > Yan Zhao <yan.y.zhao@intel.com> wrote:
> > > > > > > > > > >
> > > > > > > > > > > > Dynamic trap bar info region is a channel for QEMU and vendor driver to
> > > > > > > > > > > > communicate dynamic trap info. It is of type
> > > > > > > > > > > > VFIO_REGION_TYPE_DYNAMIC_TRAP_BAR_INFO and subtype
> > > > > > > > > > > > VFIO_REGION_SUBTYPE_DYNAMIC_TRAP_BAR_INFO.
> > > > > > > > > > > >
> > > > > > > > > > > > This region has two fields: dt_fd and trap.
> > > > > > > > > > > > When QEMU detects a device regions of this type, it will create an
> > > > > > > > > > > > eventfd and write its eventfd id to dt_fd field.
> > > > > > > > > > > > When vendor drivre signals this eventfd, QEMU reads trap field of this
> > > > > > > > > > > > info region.
> > > > > > > > > > > > - If trap is true, QEMU would search the device's PCI BAR
> > > > > > > > > > > > regions and disable all the sparse mmaped subregions (if the sparse
> > > > > > > > > > > > mmaped subregion is disablable).
> > > > > > > > > > > > - If trap is false, QEMU would re-enable those subregions.
> > > > > > > > > > > >
> > > > > > > > > > > > A typical usage is
> > > > > > > > > > > > 1. vendor driver first cuts its bar 0 into several sections, all in a
> > > > > > > > > > > > sparse mmap array. So initally, all its bar 0 are passthroughed.
> > > > > > > > > > > > 2. vendor driver specifys part of bar 0 sections to be disablable.
> > > > > > > > > > > > 3. on migration starts, vendor driver signals dt_fd and set trap to true
> > > > > > > > > > > > to notify QEMU disabling the bar 0 sections of disablable flags on.
> > > > > > > > > > > > 4. QEMU disables those bar 0 section and hence let vendor driver be able
> > > > > > > > > > > > to trap access of bar 0 registers and make dirty page tracking possible.
> > > > > > > > > > > > 5. on migration failure, vendor driver signals dt_fd to QEMU again.
> > > > > > > > > > > > QEMU reads trap field of this info region which is false and QEMU
> > > > > > > > > > > > re-passthrough the whole bar 0 region.
> > > > > > > > > > > >
> > > > > > > > > > > > Vendor driver specifies whether it supports dynamic-trap-bar-info region
> > > > > > > > > > > > through cap VFIO_PCI_DEVICE_CAP_DYNAMIC_TRAP_BAR in
> > > > > > > > > > > > vfio_pci_mediate_ops->open().
> > > > > > > > > > > >
> > > > > > > > > > > > If vfio-pci detects this cap, it will create a default
> > > > > > > > > > > > dynamic_trap_bar_info region on behalf of vendor driver with region len=0
> > > > > > > > > > > > and region->ops=null.
> > > > > > > > > > > > Vvendor driver should override this region's len, flags, rw, mmap in its
> > > > > > > > > > > > vfio_pci_mediate_ops.
> > > > > > > > > > >
> > > > > > > > > > > TBH, I don't like this interface at all. Userspace doesn't pass data
> > > > > > > > > > > to the kernel via INFO ioctls. We have a SET_IRQS ioctl for
> > > > > > > > > > > configuring user signaling with eventfds. I think we only need to
> > > > > > > > > > > define an IRQ type that tells the user to re-evaluate the sparse mmap
> > > > > > > > > > > information for a region. The user would enumerate the device IRQs via
> > > > > > > > > > > GET_IRQ_INFO, find one of this type where the IRQ info would also
> > > > > > > > > > > indicate which region(s) should be re-evaluated on signaling. The user
> > > > > > > > > > > would enable that signaling via SET_IRQS and simply re-evaluate the
> > > > > > > > > > ok. I'll try to switch to this way. Thanks for this suggestion.
> > > > > > > > > >
> > > > > > > > > > > sparse mmap capability for the associated regions when signaled.
> > > > > > > > > >
> > > > > > > > > > Do you like the "disablable" flag of sparse mmap ?
> > > > > > > > > > I think it's a lightweight way for user to switch mmap state of a whole region,
> > > > > > > > > > otherwise going through a complete flow of GET_REGION_INFO and re-setup
> > > > > > > > > > region might be too heavy.
> > > > > > > > >
> > > > > > > > > No, I don't like the disable-able flag. At what frequency do we expect
> > > > > > > > > regions to change? It seems like we'd only change when switching into
> > > > > > > > > and out of the _SAVING state, which is rare. It seems easy for
> > > > > > > > > userspace, at least QEMU, to drop the entire mmap configuration and
> > > > > > > > ok. I'll try this way.
> > > > > > > >
> > > > > > > > > re-read it. Another concern here is how do we synchronize the event?
> > > > > > > > > Are we assuming that this event would occur when a user switch to
> > > > > > > > > _SAVING mode on the device? That operation is synchronous, the device
> > > > > > > > > must be in saving mode after the write to device state completes, but
> > > > > > > > > it seems like this might be trying to add an asynchronous dependency.
> > > > > > > > > Will the write to device_state only complete once the user handles the
> > > > > > > > > eventfd? How would the kernel know when the mmap re-evaluation is
> > > > > > > > > complete. It seems like there are gaps here that the vendor driver
> > > > > > > > > could miss traps required for migration because the user hasn't
> > > > > > > > > completed the mmap transition yet. Thanks,
> > > > > > > > >
> > > > > > > > > Alex
> > > > > > > >
> > > > > > > > yes, this asynchronous event notification will cause vendor driver miss
> > > > > > > > traps. But it's supposed to be of very short period time. That's also a
> > > > > > > > reason for us to wish the re-evaluation to be lightweight. E.g. if it's
> > > > > > > > able to be finished before the first iterate, it's still safe.
> > > > > > >
> > > > > > > Making the re-evaluation lightweight cannot solve the race, it only
> > > > > > > masks it.
> > > > > > >
> > > > > > > > But I agree, the timing is not guaranteed, and so it's best for kernel
> > > > > > > > to wait for mmap re-evaluation to complete.
> > > > > > > >
> > > > > > > > migration_thread
> > > > > > > > |->qemu_savevm_state_setup
> > > > > > > > | |->ram_save_setup
> > > > > > > > | | |->migration_bitmap_sync
> > > > > > > > | | |->kvm_log_sync
> > > > > > > > | | |->vfio_log_sync
> > > > > > > > | |
> > > > > > > > | |->vfio_save_setup
> > > > > > > > | |->set_device_state(_SAVING)
> > > > > > > > |
> > > > > > > > |->qemu_savevm_state_pending
> > > > > > > > | |->ram_save_pending
> > > > > > > > | | |->migration_bitmap_sync
> > > > > > > > | | |->kvm_log_sync
> > > > > > > > | | |->vfio_log_sync
> > > > > > > > | |->vfio_save_pending
> > > > > > > > |
> > > > > > > > |->qemu_savevm_state_iterate
> > > > > > > > | |->ram_save_iterate //send pages
> > > > > > > > | |->vfio_save_iterate
> > > > > > > > ...
> > > > > > > >
> > > > > > > >
> > > > > > > > Actually, we previously let qemu trigger the re-evaluation when migration starts.
> > > > > > > > And now the reason for we to wish kernel to trigger the mmap re-evaluation is that
> > > > > > > > there're other two possible use cases:
> > > > > > > > (1) keep passing through devices when migration starts and track dirty pages
> > > > > > > > using hardware IOMMU. Then when migration is about to complete, stop the
> > > > > > > > device and start trap PCI BARs for software emulation. (we made some
> > > > > > > > changes to let device stop ahead of vcpu )
> > > > > > >
> > > > > > > How is that possible? I/O devices need to continue to work until the
> > > > > > > vCPU stops otherwise the vCPU can get blocked on the device. Maybe QEMU
> > > > > > hi Alex
> > > > > > For devices like DSA [1], it can support SVM mode. In this mode, when a
> > > > > > page fault happens, the Intel DSA device blocks until the page fault is
> > > > > > resolved, if PRS is enabled; otherwise it is reported as an error.
> > > > > >
> > > > > > Therefore, to pass through DSA into guest and do live migration with it,
> > > > > > it is desired to stop DSA before stopping vCPU, as there may be an
> > > > > > outstanding page fault to be resolved.
> > > > > >
> > > > > > During the period when DSA is stopped and vCPUs are still running, all the
> > > > > > pass-through resources are trapped and emulated by host mediation driver until
> > > > > > vCPUs stop.
> > > > >
> > > > > If the DSA is stopped and resources are trapped and emulated, then is
> > > > > the device really stopped from a QEMU perspective or has it simply
> > > > > switched modes underneath QEMU? If the device is truly stopped, then
> > > > > I'd like to understand how a vCPU doing a PIO read from the device
> > > > > wouldn't wedge the VM.
> > > > >
> > > > It doesn't matter if the device is truly stopped or not (although from
> > > > my point of view, just draining commands and keeping device running is
> > > > better as it handles live migration failure better).
> > > > PIOs also need to be trapped and emulated if a vCPU accesses them.
> > >
> > > We seem to be talking around each other here. If PIOs are trapped and
> > > emulated then the device is not "stopped" as far as QEMU is concerned,
> > > right? "Stopping" a device suggests to me that a running vCPU doing a
> > > PIO read from the device would block and cause problems in the still
> > > running VM. So I think you're suggesting some sort of mode switch in
> > > the device where direct access is disabled an emulation takes over
> > > until the vCPUs are stopped.
> >
> > sorry for this confusion.
> > yes, it's a kind of mode switch from a QEMU perspective.
> > Currently, its implementation in our local branch is like that:
> > 1. before migration thread stopping vCPUs, a migration state
> > (COMPLETING) notification is sent to vfio migration state notifier, and
> > this notifier would put device state to !RUNNING, and put all BARs to trap
> > state.
> > 2. in the kernel, when device state is set to !RUNNING, draining all
> > pending device requests, and starts emulation.
> >
> > This implementation has two issues:
> > 1. it requires hardcode in QEMU to put all BARs trapped and the time
> > spending on revoking mmaps is not necessary for devices that do not need it.
> > 2. !RUNNING state here is not accurate and it will confuse vendor
> > drivers who stop devices after vCPUs stop.
> >
> > For the 2nd issue, I think we can propose a new device state like
> > PRE-STOPPING.
>
> Yes, this is absolutely abusing the !RUNNING state, if the device is
> still processing accesses by the vCPU, it's still running.
>
> > But for the 1st issue, not sure how to fix it right now.
> > Maybe we can still add an asynchronous kernel notification and wait until
> > QEMU have switched the region mmap state?
>
> It seems like you're preemptively trying to optimize the SAVING state
> before we even have migration working. Shouldn't SAVING be the point
> at which you switch to trapping the device in order to track it?
> Thanks,

But for some devices, start trapping on entering SAVING state is too
early. They don't really need the trapping until PRE_STOPPING stage.
E.g. for DSA, it can get dirty pages without trapping. The intention for
it to enter trap is not for SAVING, but for emulation.

Thanks
Yan