Re: [PATCH v2 18/30] virtio_fs, dax: Set up virtio_fs dax_device

[Christian Borntraeger]

On 18.07.19 16:30, Dan Williams wrote:
> On Thu, Jul 18, 2019 at 6:15 AM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>>
>> On Wed, Jul 17, 2019 at 07:27:25PM +0200, Halil Pasic wrote:
>>> On Wed, 15 May 2019 15:27:03 -0400
>>> Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>>>
>>>> From: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
>>>>
>>>> Setup a dax device.
>>>>
>>>> Use the shm capability to find the cache entry and map it.
>>>>
>>>> The DAX window is accessed by the fs/dax.c infrastructure and must have
>>>> struct pages (at least on x86).  Use devm_memremap_pages() to map the
>>>> DAX window PCI BAR and allocate struct page.
>>>>
>>>
>>> Sorry for being this late. I don't see any more recent version so I will
>>> comment here.
>>>
>>> I'm trying to figure out how is this supposed to work on s390. My concern
>>> is, that on s390 PCI memory needs to be accessed by special
>>> instructions. This is taken care of by the stuff defined in
>>> arch/s390/include/asm/io.h. E.g. we 'override' __raw_writew so it uses
>>> the appropriate s390 instruction. However if the code does not use the
>>> linux abstractions for accessing PCI memory, but assumes it can be
>>> accessed like RAM, we have a problem.
>>>
>>> Looking at this patch, it seems to me, that we might end up with exactly
>>> the case described. For example AFAICT copy_to_iter() (3) resolves to
>>> the function in lib/iov_iter.c which does not seem to cater for s390
>>> oddities.
>>>
>>> I didn't have the time to investigate this properly, and since virtio-fs
>>> is virtual, we may be able to get around what is otherwise a
>>> limitation on s390. My understanding of these areas is admittedly
>>> shallow, and since I'm not sure I'll have much more time to
>>> invest in the near future I decided to raise concern.
>>>
>>> Any opinions?
>>
>> Hi Halil,
>>
>> I don't understand s390 and how PCI works there as well. Is there any
>> other transport we can use there to map IO memory directly and access
>> using DAX?
>>
>> BTW, is DAX supported for s390.
>>
>> I am also hoping somebody who knows better can chip in. Till that time,
>> we could still use virtio-fs on s390 without DAX.
> 
> s390 has so-called "limited" dax support, see CONFIG_FS_DAX_LIMITED.
> In practice that means that support for PTE_DEVMAP is missing which
> means no get_user_pages() support for dax mappings. Effectively it's
> only useful for execute-in-place as operations like fork() and ptrace
> of dax mappings will fail.


This is only true for the dcssblk device driver (drivers/s390/block/dcssblk.c
and arch/s390/mm/extmem.c). 

For what its worth, the dcssblk looks to Linux like normal memory (just above the
previously detected memory) that can be used like normal memory. In previous time
we even had struct pages for this memory - this was removed long ago (when it was
still xip) to reduce the memory footprint for large dcss blocks and small memory
guests.
Can the CONFIG_FS_DAX_LIMITED go away if we have struct pages for that memory?

Now some observations: 
- dcssblk is z/VM only (not KVM)
- Setting CONFIG_FS_DAX_LIMITED globally as a Kconfig option depending on wether
  a device driver is compiled in or not seems not flexible enough in case if you
  have device driver that does have struct pages and another one that doesn't
- I do not see a reason why we should not be able to map anything from QEMU
  into the guest real memory via an additional KVM memory slot. 
  We would need to handle that in the guest somehow (and not as a PCI bar),
  register this with struct pages etc.
- we must then look how we can create the link between the guest memory and the
  virtio-fs driver. For virtio-ccw we might be able to add a new ccw command or
  whatever. Maybe we could also piggy-back on some memory hotplug work from David
  Hildenbrand (add cc).

Regarding limitations on the platform:
- while we do have PCI, the virtio devices are usually plugged via the ccw bus.
  That implies no PCI bars. I assume you use those PCI bars only to implicitely 
  have the location of the shared memory
  Correct?
- no real memory mapped I/O. Instead there are instructions that work on the mmio.
  As I understand things, this is of no concern regarding virtio-fs as you do not
  need mmio in the sense that a memory access of the guest to such an address 
  triggers an exit. You just need the shared memory as a mean to have the data
  inside the guest. Any notification is done via normal virtqueue mechanisms
  Correct?


Adding Heiko, maybe he remembers some details of the dcssblk/xip history.