Re: [PATCH v2 10/20] vfio/common: Record DMA mapped IOVA ranges

[Alex Williamson]

On Thu, 23 Feb 2023 10:37:10 +0000
Joao Martins <joao.m.martins@oracle.com> wrote:

> On 22/02/2023 22:10, Alex Williamson wrote:
> > On Wed, 22 Feb 2023 19:49:05 +0200
> > Avihai Horon <avihaih@nvidia.com> wrote:  
> >> From: Joao Martins <joao.m.martins@oracle.com>
> >> @@ -612,6 +665,16 @@ static int vfio_dma_map(VFIOContainer *container, 
> >> hwaddr iova,
> >>          .iova = iova,
> >>          .size = size,
> >>      };
> >> +    int ret;
> >> +
> >> +    ret = vfio_record_mapping(container, iova, size, readonly);
> >> +    if (ret) {
> >> +        error_report("vfio: Failed to record mapping, iova: 0x%" 
> >> HWADDR_PRIx
> >> +                     ", size: 0x" RAM_ADDR_FMT ", ret: %d (%s)",
> >> +                     iova, size, ret, strerror(-ret));
> >> +
> >> +        return ret;
> >> +    }  
> > 
> > Is there no way to replay the mappings when a migration is started?
> > This seems like a horrible latency and bloat trade-off for the
> > possibility that the VM might migrate and the device might support
> > these features.  Our performance with vIOMMU is already terrible, I
> > can't help but believe this makes it worse.  Thanks,
> >   
> 
> It is a nop if the vIOMMU is being used (entries in container->giommu_list) as
> that uses a max-iova based IOVA range. So this is really for iommu identity
> mapping and no-VIOMMU.

Ok, yes, there are no mappings recorded for any containers that have a
non-empty giommu_list.

> We could replay them if they were tracked/stored anywhere.

Rather than piggybacking on vfio_memory_listener, why not simply
register a new MemoryListener when migration is started?  That will
replay all the existing ranges and allow tracking to happen separate
from mapping, and only when needed.

> I suppose we could move the vfio_devices_all_device_dirty_tracking() into this
> patch and then conditionally call this vfio_{record,erase}_mapping() in case 
> we
> are passing through a device that doesn't have live-migration support? Would
> that address the impact you're concerned wrt to non-live-migrateable devices?
> 
> On the other hand, the PCI device hotplug hypothetical even makes this a bit
> complicated as we can still attempt to hotplug a device before migration is 
> even
> attempted. Meaning that we start with live-migrateable devices, and we added 
> the
> tracking, up to hotpluging a device without such support (adding a blocker)
> leaving the mappings there with no further use. So it felt simpler to just 
> track
> always and avoid any mappings recording if the vIOMMU is in active use?

My preference would be that there's no runtime overhead for migration
support until a migration is initiated.  I currently don't see why we
can't achieve that by dynamically adding a new MemoryListener around
migration for that purpose.  Do you?  Thanks,

Alex