netdev - Re: [PATCH v14 vfio 6/8] vfio/pds: Add support for dirty page tracking

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Date: Wed, 9 Aug 2023 11:33:00 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Brett Creeley <bcreeley@....com>
Cc: Brett Creeley <brett.creeley@....com>, kvm@...r.kernel.org,
 netdev@...r.kernel.org, jgg@...dia.com, yishaih@...dia.com,
 shameerali.kolothum.thodi@...wei.com, kevin.tian@...el.com,
 horms@...nel.org, shannon.nelson@....com
Subject: Re: [PATCH v14 vfio 6/8] vfio/pds: Add support for dirty page
 tracking

On Wed, 9 Aug 2023 08:44:44 -0700
Brett Creeley <bcreeley@....com> wrote:

> On 8/8/2023 3:27 PM, Alex Williamson wrote:
> > Caution: This message originated from an External Source. Use proper caution when opening attachments, clicking links, or responding.
> > 
> > 
> > On Mon, 7 Aug 2023 13:57:53 -0700
> > Brett Creeley <brett.creeley@....com> wrote:
> > ...  
> >> +static int pds_vfio_dirty_enable(struct pds_vfio_pci_device *pds_vfio,
> >> +                              struct rb_root_cached *ranges, u32 nnodes,
> >> +                              u64 *page_size)
> >> +{
> >> +     struct pci_dev *pdev = pds_vfio->vfio_coredev.pdev;
> >> +     struct device *pdsc_dev = &pci_physfn(pdev)->dev;
> >> +     struct pds_vfio_dirty *dirty = &pds_vfio->dirty;
> >> +     u64 region_start, region_size, region_page_size;
> >> +     struct pds_lm_dirty_region_info *region_info;
> >> +     struct interval_tree_node *node = NULL;
> >> +     u8 max_regions = 0, num_regions;
> >> +     dma_addr_t regions_dma = 0;
> >> +     u32 num_ranges = nnodes;
> >> +     u32 page_count;
> >> +     u16 len;
> >> +     int err;
> >> +
> >> +     dev_dbg(&pdev->dev, "vf%u: Start dirty page tracking\n",
> >> +             pds_vfio->vf_id);
> >> +
> >> +     if (pds_vfio_dirty_is_enabled(pds_vfio))
> >> +             return -EINVAL;
> >> +
> >> +     /* find if dirty tracking is disabled, i.e. num_regions == 0 */
> >> +     err = pds_vfio_dirty_status_cmd(pds_vfio, 0, &max_regions,
> >> +                                     &num_regions);
> >> +     if (err < 0) {
> >> +             dev_err(&pdev->dev, "Failed to get dirty status, err %pe\n",
> >> +                     ERR_PTR(err));
> >> +             return err;
> >> +     } else if (num_regions) {
> >> +             dev_err(&pdev->dev,
> >> +                     "Dirty tracking already enabled for %d regions\n",
> >> +                     num_regions);
> >> +             return -EEXIST;
> >> +     } else if (!max_regions) {
> >> +             dev_err(&pdev->dev,
> >> +                     "Device doesn't support dirty tracking, max_regions %d\n",
> >> +                     max_regions);
> >> +             return -EOPNOTSUPP;
> >> +     }
> >> +
> >> +     /*
> >> +      * Only support 1 region for now. If there are any large gaps in the
> >> +      * VM's address regions, then this would be a waste of memory as we are
> >> +      * generating 2 bitmaps (ack/seq) from the min address to the max
> >> +      * address of the VM's address regions. In the future, if we support
> >> +      * more than one region in the device/driver we can split the bitmaps
> >> +      * on the largest address region gaps. We can do this split up to the
> >> +      * max_regions times returned from the dirty_status command.
> >> +      */  
> > 
> > Isn't this a pretty unfortunately limitation given QEMU makes a 1TB
> > hole on AMD hosts?  Or maybe I misunderstand.
> > 
> > https://gitlab.com/qemu-project/qemu/-/commit/8504f129450b909c88e199ca44facd35d38ba4de
> > 
> > Thanks,
> > Alex
> >   
> 
> Yes, this is currently an unfortunate limitation. However, our device is 
> flexible enough to support >1 regions. There has been some work in this 
> area, but we aren't quite there yet. The goal was to get this initial 
> support accepted and submit follow on work to support >1 regions.

Ok, good that this is temporary.

Shameer, Kevin, Jason, Yishai, I'm hoping one or more of you can
approve this series as well.  Thanks,

Alex