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

On 22.07.19 12:56, Dr. David Alan Gilbert wrote: > * Christian Borntraeger (borntraeger(a)de.ibm.com) wrote: >> >> >> On 18.07.19 16:30, Dan Williams wrote: >>> On Thu, Jul 18, 2019 at 6:15 AM Vivek Goyal <vgoyal(a)redhat.com&gt; 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(a)redhat.com&gt; wrote: >>>>> >>>>>> From: Stefan Hajnoczi <stefanha(a)redhat.com&gt; >>>>>> >>>>>> 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? > > 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)

> >> - 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.