Re: [PATCH V7 6/9] iommufd: pfn reader for file mappings

[Alexey Kardashevskiy <aik@amd.com>]

On 15/11/24 03:17, Jason Gunthorpe wrote:
> On Thu, Nov 14, 2024 at 03:03:36PM +1100, Alexey Kardashevskiy wrote:
>>>> I though KVM had its own private interface to guestmemfd?
>>>
>>> At the moment it is essentially this:
>>>
>>> folio = filemap_grab_folio(file_inode(file)->i_mapping, index);
>>
>> Turns out the complete sequence is:
>>
>> folio = filemap_grab_folio(file_inode(file)->i_mapping, index);
>> folio_add_pins(folio, 1);
>> folio_put(folio);
>>
>> so then unpin_user_page_range_dirty_lock() => gup_fast_unpin_user_pages() =>
>> gup_put_folio() work properly.
> 
> I'm surprised given how forceful people were that pinning should not
> be done there. Please confirm with guestmemfd experts..
> 
>> Later on, hopefully, guest_memfd will developed ability to mmap() and then
>> the existing iommufd mapping API will "just work", I suppose. I'll carry on
>> the above for now. Thanks,
> 
> I think that is not possible, the entire point of guest memfd is that
> private memory is never mapped.

It was like that but recently the approach changed to allow mappings. I 
do not think the interface is finalized yet though.

> For the AMD case we still need to program the address of private
> memory into the hypervisor iommu page tables, so the FD path will have
> to used to make that connection.

Right, using it now in my working tree, with this new vdevice thingy 
https://github.com/aik/linux/commits/tsm/
Thanks,

-- 
Alexey