Subject: Re: [PATCH V7 vfio 07/10] vfio/mlx5: Create and destroy page tracker object

From: Jason Gunthorpe <[email-masked]>
Date: Wed, 6 Sep 2023 08:51:26 -0300

On Wed, Sep 06, 2023 at 10:55:26AM +0200, Cédric Le Goater wrote:

> > +	WARN_ON(node);
> > +	log_addr_space_size = ilog2(total_ranges_len);
> > +	if (log_addr_space_size <
> > +	    (MLX5_CAP_ADV_VIRTUALIZATION(mdev, pg_track_log_min_addr_space)) ||
> > +	    log_addr_space_size >
> > +	    (MLX5_CAP_ADV_VIRTUALIZATION(mdev, pg_track_log_max_addr_space))) {
> > +		err = -EOPNOTSUPP;
> > +		goto out;
> > +	}
> 
> 
> We are seeing an issue with dirty page tracking when doing migration
> of an OVMF VM guest. The vfio-pci variant driver for the MLX5 VF
> device complains when dirty page tracking is initialized from QEMU :
> 
>   qemu-kvm: 0000:b1:00.2: Failed to start DMA logging, err -95 (Operation not supported)
> 
> The 64-bit computed range is  :
> 
>   vfio_device_dirty_tracking_start nr_ranges 2 32:[0x0 - 0x807fffff], 64:[0x100000000 - 0x3838000fffff]
> 
> which seems to be too large for the HW. AFAICT, the MLX5 HW has a 42
> bits address space limitation for dirty tracking (min is 12). Is it a
> FW tunable or a strict limitation ?

It would be good to explain where this is coming from, all devices
need to make some decision on what address space ranges to track and I
would say 2^42 is already pretty generous limit..

Can we go the other direction and reduce the ranges qemu is interested
in?

Jason