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From: Christian Borntraeger <borntraeger@de.ibm.com>
To: Dan Williams <dan.j.williams@intel.com>, Vivek Goyal <vgoyal@redhat.com>
Cc: Halil Pasic <pasic@linux.ibm.com>,
	Collin Walling <walling@linux.ibm.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Sebastian Ott <sebott@linux.ibm.com>,
	KVM list <kvm@vger.kernel.org>,
	Miklos Szeredi <miklos@szeredi.hu>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Steven Whitehouse <swhiteho@redhat.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v2 18/30] virtio_fs, dax: Set up virtio_fs dax_device
Date: Mon, 22 Jul 2019 12:51:49 +0200
Message-ID: <c519011e-1df3-3f35-8582-2cb58367ff8a@de.ibm.com> (raw)
In-Reply-To: <CAPcyv4i+2nKJYqkbrdm3hWcjaMYkCKUxqLBq96HOZe6xOZzGGg@mail.gmail.com>

On 18.07.19 16:30, Dan Williams wrote:
> On Thu, Jul 18, 2019 at 6:15 AM Vivek Goyal <vgoyal@redhat.com> 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@redhat.com> wrote:
>>>> From: Stefan Hajnoczi <stefanha@redhat.com>
>>>> 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
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
- 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

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

  reply index

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-15 19:26 [PATCH v2 00/30] [RFC] virtio-fs: shared file system for virtual machines Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 01/30] fuse: delete dentry if timeout is zero Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 02/30] fuse: Clear setuid bit even in cache=never path Vivek Goyal
2019-05-20 14:41   ` Miklos Szeredi
2019-05-20 14:44     ` Miklos Szeredi
2019-05-20 20:25       ` Nikolaus Rath
2019-05-21 15:01     ` Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 03/30] fuse: Use default_file_splice_read for direct IO Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 04/30] fuse: export fuse_end_request() Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 05/30] fuse: export fuse_len_args() Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 06/30] fuse: Export fuse_send_init_request() Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 07/30] fuse: export fuse_get_unique() Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 08/30] fuse: extract fuse_fill_super_common() Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 09/30] fuse: add fuse_iqueue_ops callbacks Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 10/30] fuse: Separate fuse device allocation and installation in fuse_conn Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 11/30] virtio_fs: add skeleton virtio_fs.ko module Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 12/30] dax: remove block device dependencies Vivek Goyal
2019-05-16  0:21   ` Dan Williams
2019-05-16 10:07     ` Stefan Hajnoczi
2019-05-16 14:23     ` Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 13/30] dax: Pass dax_dev to dax_writeback_mapping_range() Vivek Goyal
2019-05-15 19:26 ` [PATCH v2 14/30] virtio: Add get_shm_region method Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 15/30] virtio: Implement get_shm_region for PCI transport Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 16/30] virtio: Implement get_shm_region for MMIO transport Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 17/30] fuse, dax: add fuse_conn->dax_dev field Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 18/30] virtio_fs, dax: Set up virtio_fs dax_device Vivek Goyal
2019-07-17 17:27   ` Halil Pasic
2019-07-18  9:04     ` Cornelia Huck
2019-07-18 11:20       ` Halil Pasic
2019-07-18 14:47         ` Cornelia Huck
2019-07-18 13:15     ` Vivek Goyal
2019-07-18 14:30       ` Dan Williams
2019-07-22 10:51         ` Christian Borntraeger [this message]
2019-07-22 10:56           ` Dr. David Alan Gilbert
2019-07-22 11:20             ` Christian Borntraeger
2019-07-22 11:43               ` Cornelia Huck
2019-07-22 12:00                 ` Christian Borntraeger
2019-07-22 12:08                   ` David Hildenbrand
2019-07-29 13:20                     ` Stefan Hajnoczi
2019-05-15 19:27 ` [PATCH v2 19/30] fuse: Keep a list of free dax memory ranges Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 20/30] fuse: Introduce setupmapping/removemapping commands Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 21/30] fuse, dax: Implement dax read/write operations Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 22/30] fuse, dax: add DAX mmap support Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 23/30] fuse: Define dax address space operations Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 24/30] fuse, dax: Take ->i_mmap_sem lock during dax page fault Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 25/30] fuse: Maintain a list of busy elements Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 26/30] fuse: Add logic to free up a memory range Vivek Goyal
     [not found]   ` <CAN+Pk99SNKSf+GjSQUUWt_eu1fSjTy_ByUOEQUXHi8zNqXY1zA@mail.gmail.com>
2019-05-20 12:53     ` Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 27/30] fuse: Release file in process context Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 28/30] fuse: Reschedule dax free work if too many EAGAIN attempts Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 29/30] fuse: Take inode lock for dax inode truncation Vivek Goyal
2019-05-15 19:27 ` [PATCH v2 30/30] virtio-fs: Do not provide abort interface in fusectl Vivek Goyal

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