Re: [RFC PATCH kvmtool 09/10] vfio/iommufd: Add viommu and vdevice objects

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Will Deacon <will@kernel.org> writes:

> On Mon, Aug 04, 2025 at 11:33:27PM +0100, Suzuki K Poulose wrote:
>> On 24/07/2025 15:09, Aneesh Kumar K.V wrote:
>> > Will Deacon <will@kernel.org> writes:
>> > > On Sun, May 25, 2025 at 01:19:15PM +0530, Aneesh Kumar K.V (Arm) wrote:
>> > > > +	dev_num = vdev->dev_hdr.dev_num;
>> > > > +	/* kvmtool only do 0 domain, 0 bus and 0 function devices. */
>> > > > +	guest_bdf = (0ULL << 32) | (0 << 16) | dev_num << 11 | (0 << 8);
>> > > 
>> > > I don't understand this. Shouldn't the BDF correspond to the virtual
>> > > configuration space? That's not allocated until later, but just going
>> > > with 0 isn't going to work.
>> > > 
>> > > What am I missing?
>> > > 
>> > 
>> > As I understand it, kvmtool supports only bus 0 and does not allow
>> > multifunction devices. Based on that, I derived the guest BDF as follows
>> > (correcting what was wrong in the original patch):
>> > 
>> > guest_bdf = (0ULL << 16) | (0 << 8) | dev_num << 3 | (0 << 0);
>> > 
>> > Are you suggesting that this approach is incorrect, and that we can use
>> > a bus number other than 0?
>> 
>> To put this other way, the emulation of the configuration space is based
>> on the "dev_num". i.e., CFG address is converted to the offset and
>> mapped to the "dev_num". So I think what we have here is correct.
>
> My point is that 'dev_num' isn't allocated until vfio_pci_setup_device(),
> which is called from __iommufd_configure_device() _after_  we've called
> iommufd_alloc_s1bypass_hwpt().
>
> So I don't see how this works. You have to allocate the virtual config
> space before you can allocate the virtual device with iommufd.
>

I did fix that in https://lore.kernel.org/all/yq5att31brz2.fsf@kernel.org/ 

-aneesh