On Mon, 22 Jul 2019 13:20:18 +0200 Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote: > On 22.07.19 12:56, Dr. David Alan Gilbert wrote: > > * Christian Borntraeger (borntraeger@xxxxxxxxxx) wrote: > >> > >> > >> 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. You mean for ccw, right? I don't think we want pci to behave differently than everywhere else. > >> - 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? > > > > Right. > > So in essence we just have to provide a vm_get_shm_region callback in the virtio-ccw > guest code? > > How many regions do we have to support? One region per device? Or many? > Even if we need more, this should be possible with a 2 new CCWs, e.g READ_SHM_BASE(id) > and READ_SHM_SIZE(id) I'd just add a single CCW with a control block containing id and size. The main issue is where we put those regions, and what happens if we use both virtio-pci and virtio-ccw on the same machine. > > > > > >> - 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? > > > > Yep. >
