Re: [Qemu-devel] [PATCH] vfio: Fix handling VFIO_IOMMU_GET_INFO results

[Alex Williamson]

On Fri, 2015-11-13 at 12:33 +0300, Pavel Fedin wrote:
>  Hello!
> 
> > >  If we fix qemu, it will automatically start working with all
> > > available kernels which are there in the wild. If we fix kernel, older
> > > versions will still not work, however they can.
> > >  That's why i think that we should adapt qemu to what already exists.
> > > But, well, you are The Boss, so you can just say "i don't care". So,
> > > just let me now if you strongly disagree with this.
> > 
> > I do care, in fact I care enough about the ABI that I'm suggesting what
> > I think is the correct fix rather than taking the quick and dirty
> > solution.  It's an unfortunate bug, but it's not worth changing the ABI
> > and removing the kernel's ability to indicate whether the pgsize bitmap
> > field is valid IMO.
> 
>  Ok, i see your point...
>  But what about fix, which would work both with future kernels, which
> do provide this flag, as well as would be compatible with already
> existing kernels, which set flags == 0?
>  We could check for ((info.flags == 0) || (info.flags &
> VFIO_IOMMU_INFO_PGSIZES)). This would conform to both behaviors:
>  a) All current kernels set flags = 0 and report page sizes.
>  b) Some future kernels could have set some flags, but not reported
> page sizes and not set VFIO_IOMMI_PGSIZES
> 
>  What would you say about this? Yes, this would be a "workaround"
> instead of "fix".

You haven't presented a compelling reason to do this.  What is the case
that you're thinking of that assuming PAGE_SIZE doesn't work or at least
provide some degree of backwards compatibility?  QEMU has been doing
this all along.  Thanks,

Alex