> From: Alex Williamson [mailto:alex.william...@redhat.com] > Sent: Wednesday, June 28, 2017 3:45 AM > > On Tue, 27 Jun 2017 08:56:01 +0000 > "Zhang, Yulei" <yulei.zh...@intel.com> wrote: > > > > -----Original Message----- > > > From: Qemu-devel [mailto:qemu-devel- > > > bounces+yulei.zhang=intel....@nongnu.org] On Behalf Of Alex > Williamson > > > Sent: Tuesday, June 27, 2017 4:19 AM > > > To: Zhang, Yulei <yulei.zh...@intel.com> > > > Cc: Tian, Kevin <kevin.t...@intel.com>; joonas.lahti...@linux.intel.com; > > > qemu-devel@nongnu.org; zhen...@linux.intel.com; Zheng, Xiao > > > <xiao.zh...@intel.com>; Wang, Zhi A <zhi.a.w...@intel.com> > > > Subject: Re: [Qemu-devel] [RFC 5/5] vifo: introduce new VFIO ioctl > > > VFIO_DEVICE_PCI_GET_DIRTY_BITMAP > > > > > > On Tue, 4 Apr 2017 18:28:04 +0800 > > > Yulei Zhang <yulei.zh...@intel.com> wrote: > > > > > > > New VFIO ioctl VFIO_DEVICE_PCI_GET_DIRTY_BITMAP is used to sync > the > > > > pci device dirty pages during the migration. > > > > > > If this needs to exist, it needs a lot more documentation. Why is this > > > a PCI specific device ioctl? Couldn't any vfio device need this? > > > > > > > Signed-off-by: Yulei Zhang <yulei.zh...@intel.com> > > > > --- > > > > hw/vfio/pci.c | 32 ++++++++++++++++++++++++++++++++ > > > > hw/vfio/pci.h | 2 ++ > > > > linux-headers/linux/vfio.h | 14 ++++++++++++++ > > > > 3 files changed, 48 insertions(+) > > > > > > > > diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c > > > > index 833cd90..64c851f 100644 > > > > --- a/hw/vfio/pci.c > > > > +++ b/hw/vfio/pci.c > > > > @@ -32,6 +32,7 @@ > > > > #include "pci.h" > > > > #include "trace.h" > > > > #include "qapi/error.h" > > > > +#include "exec/ram_addr.h" > > > > > > > > #define MSIX_CAP_LENGTH 12 > > > > > > > > @@ -39,6 +40,7 @@ static void vfio_disable_interrupts(VFIOPCIDevice > > > *vdev); > > > > static void vfio_mmap_set_enabled(VFIOPCIDevice *vdev, bool > enabled); > > > > static VMStateDescription vfio_pci_vmstate; > > > > static void vfio_vm_change_state_handler(void *pv, int running, > RunState > > > state); > > > > +static void vfio_log_sync(MemoryListener *listener, > > > MemoryRegionSection *section); > > > > > > > > /* > > > > * Disabling BAR mmaping can be slow, but toggling it around INTx can > > > > @@ -2869,6 +2871,11 @@ static void vfio_realize(PCIDevice *pdev, > Error > > > **errp) > > > > vfio_setup_resetfn_quirk(vdev); > > > > > qemu_add_vm_change_state_handler(vfio_vm_change_state_handler, > > > vdev); > > > > > > > > + vdev->vfio_memory_listener = (MemoryListener) { > > > > + .log_sync = vfio_log_sync, > > > > + }; > > > > + memory_listener_register(&vdev->vfio_memory_listener, > > > &address_space_memory); > > > > + > > > > return; > > > > > > > > out_teardown: > > > > @@ -2964,6 +2971,7 @@ static void > vfio_vm_change_state_handler(void > > > *pv, int running, RunState state) > > > > if (ioctl(vdev->vbasedev.fd, VFIO_DEVICE_PCI_STATUS_SET, > vfio_status)) > > > { > > > > error_report("vfio: Failed to %s device\n", running ? "start" : > "stop"); > > > > } > > > > + vdev->device_stop = running ? false : true; > > > > g_free(vfio_status); > > > > } > > > > > > > > @@ -3079,6 +3087,30 @@ static int vfio_device_get(QEMUFile *f, void > *pv, > > > size_t size, VMStateField *fie > > > > return 0; > > > > } > > > > > > > > +static void vfio_log_sync(MemoryListener *listener, > > > MemoryRegionSection *section) > > > > +{ > > > > + VFIOPCIDevice *vdev = container_of(listener, struct VFIOPCIDevice, > > > vfio_memory_listener); > > > > + > > > > + if (vdev->device_stop) { > > > > + struct vfio_pci_get_dirty_bitmap *d; > > > > + ram_addr_t size = int128_get64(section->size); > > > > + unsigned long page_nr = size >> TARGET_PAGE_BITS; > > > > + unsigned long bitmap_size = (BITS_TO_LONGS(page_nr) + 1) * > > > sizeof(unsigned long); > > > > + d = g_malloc0(sizeof(*d) + bitmap_size); > > > > + d->start_addr = section->offset_within_address_space; > > > > + d->page_nr = page_nr; > > > > + > > > > + if (ioctl(vdev->vbasedev.fd, > VFIO_DEVICE_PCI_GET_DIRTY_BITMAP, d)) > > > { > > > > + error_report("vfio: Failed to fetch dirty pages for > > > > migration\n"); > > > > + goto exit; > > > > + } > > > > + cpu_physical_memory_set_dirty_lebitmap((unsigned long*)&d- > > > >dirty_bitmap, d->start_addr, d->page_nr); > > > > + > > > > +exit: > > > > + g_free(d); > > > > + } > > > > +} > > > > + > > > > static void vfio_instance_init(Object *obj) > > > > { > > > > PCIDevice *pci_dev = PCI_DEVICE(obj); > > > > diff --git a/hw/vfio/pci.h b/hw/vfio/pci.h > > > > index bd98618..984391d 100644 > > > > --- a/hw/vfio/pci.h > > > > +++ b/hw/vfio/pci.h > > > > @@ -144,6 +144,8 @@ typedef struct VFIOPCIDevice { > > > > bool no_kvm_intx; > > > > bool no_kvm_msi; > > > > bool no_kvm_msix; > > > > + bool device_stop; > > > > + MemoryListener vfio_memory_listener; > > > > } VFIOPCIDevice; > > > > > > > > uint32_t vfio_pci_read_config(PCIDevice *pdev, uint32_t addr, int len); > > > > diff --git a/linux-headers/linux/vfio.h b/linux-headers/linux/vfio.h > > > > index fa17848..aa73ee1 100644 > > > > --- a/linux-headers/linux/vfio.h > > > > +++ b/linux-headers/linux/vfio.h > > > > @@ -502,6 +502,20 @@ struct vfio_pci_status_set{ > > > > > > > > #define VFIO_DEVICE_PCI_STATUS_SET _IO(VFIO_TYPE, VFIO_BASE + > 14) > > > > > > > > +/** > > > > + * VFIO_DEVICE_PCI_GET_DIRTY_BITMAP - _IOW(VFIO_TYPE, > VFIO_BASE + > > > 15, > > > > + * struct vfio_pci_get_dirty_bitmap) > > > > + * > > > > + * Return: 0 on success, -errno on failure. > > > > + */ > > > > +struct vfio_pci_get_dirty_bitmap{ > > > > + __u64 start_addr; > > > > + __u64 page_nr; > > > > + __u8 dirty_bitmap[]; > > > > +}; > > > > + > > > > +#define VFIO_DEVICE_PCI_GET_DIRTY_BITMAP _IO(VFIO_TYPE, > VFIO_BASE > > > + 15) > > > > + > > > > > > Dirty since when? Since the last time we asked? Since the device was > > > stopped? Why is anything dirtied after the device is stopped? Is this > > > any pages the device has ever touched? Thanks, > > > > > > Alex > > Dirty since the device start operation and before it was stopped. We track > > down all the guest pages that device was using before it was stopped, and > > leverage this dirty bitmap for page sync during migration. > > I don't understand how this is useful or efficient. This implies that > the device is always tracking dirtied pages even when we don't care > about migration. Don't we want to enable dirty logging at some point > and track dirty pages since then? Otherwise we can just assume the > device dirties all pages and get rid of this ioctl. Thanks, >
Agree. Regarding to interface definition we'd better follow general dirty logging scheme as Alex pointed out, possibly through another ioctl cmd to enable/disable logging. However vendor specific implementation may choose to ignore the cmd while always tracking dirty pages, as on Intel Processor Graphics. Below is some background. CPU dirty logging is done through either CPU page fault or HW dirty bit logging (e.g. Intel PML). However there is a gap in DMA side today. DMA page fault requires both IOMMU and device support (through PCI ATS/PRS), which is not widely available today and mostly for special types of workloads (e.g. Shared Virtual Memory). Regarding to dirty bit, at least VTd doesn't support it today. So the alternative option is to rely on mediation layer to track the dirty pages, since workload submissions on vGPU are mediated. It's feasible for simple devices such as NIC, which has a clear definition of descriptors so it's easy to scan and capture which pages will be dirtied. However doing same thing for complex GPU (meaning to scan all GPU commands, shader instructions, indirect structures, etc.) is weigh too complex and insufficient. Today we only scan privileged commands for security purpose which is only a very small portion of all possible cmd set. Then in reality we choose a simplified approach. instead of tracking incrementally dirtied pages since last query, we treat all pages which are currently mapped in GPU page tables as dirtied. To avoid overhead of walking global page table (GGTT) and all active per-process page tables (PPGTTs) upon query, we choose to always maintain a bitmap which is updated when mediating guest updates to those GTT entries. It adds negligible overhead at run-time since those operations are already mediated. Every time when Qemu tries to query dirty map, it likely will get a similar large dirty bitmap (not exactly same since GPU page tables are being changed) then will exit iterative memory copy very soon, which will ends up like below: 1st round: Qemu copies all the memory (say 4GB) to another machine 2nd round: Qemu queries vGPU dirty map (usually in several hundreds of MBs) and combine with CPU dirty map to copy 3rd round: Qemu will get similar size of dirty pages then exit the pre-copy phase since dirty set doesn't converge Although it's not that efficient, no need to stop service for whole 4GB memory copy still saves a lot. In our measurement the service shutdown time is ~300ms over 10Gb link when running 3D benchmarks (e.g. 3DMark, Heaven, etc.) and media transcoding workloads, while copying whole system memory may easily take seconds to trigger TDR. though service shutdown time is bigger than usual server-based scenario, it's somehow OK for interactive usages (e.g. VDI) or offline transcoding usages. You may take a look at our demo at: https://www.youtube.com/watch?v=y2SkU5JODIY In a nutshell, our current dirty logging implementation is a bit awkward due to arch limitation, but it does work well for some scenarios. Most importantly I agree we should design interface in a more general way to enable/disable dirty logging as stated earlier. Hope above makes the whole background clearer. :-) Thanks Kevin
