Message-ID: <20180719114859.29348ed4@doriath>
Date:   Thu, 19 Jul 2018 11:48:59 -0400
From:   Luiz Capitulino <lcapitulino@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>,
        Pankaj Gupta <pagupta@...hat.com>, kwolf@...hat.com,
        haozhong zhang <haozhong.zhang@...el.com>, jack@...e.cz,
        xiaoguangrong eric <xiaoguangrong.eric@...il.com>,
        kvm@...r.kernel.org, riel@...riel.com, linux-nvdimm@...1.01.org,
        ross zwisler <ross.zwisler@...el.com>,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        hch@...radead.org, imammedo@...hat.com, mst@...hat.com,
        niteshnarayanlal@...mail.com, pbonzini@...hat.com,
        dan j williams <dan.j.williams@...el.com>, nilal@...hat.com
Subject: Re: [Qemu-devel] [RFC v3] qemu: Add virtio pmem device

On Thu, 19 Jul 2018 15:58:20 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 19.07.2018 14:16, Stefan Hajnoczi wrote:
> > On Thu, Jul 19, 2018 at 01:48:13AM -0400, Pankaj Gupta wrote:  
> >>  
> >>>  
> >>>>  This patch adds virtio-pmem Qemu device.
> >>>>
> >>>>  This device presents memory address range information to guest
> >>>>  which is backed by file backend type. It acts like persistent
> >>>>  memory device for KVM guest. Guest can perform read and persistent
> >>>>  write operations on this memory range with the help of DAX capable
> >>>>  filesystem.
> >>>>
> >>>>  Persistent guest writes are assured with the help of virtio based
> >>>>  flushing interface. When guest userspace space performs fsync on
> >>>>  file fd on pmem device, a flush command is send to Qemu over VIRTIO
> >>>>  and host side flush/sync is done on backing image file.
> >>>>
> >>>> Changes from RFC v2:
> >>>> - Use aio_worker() to avoid Qemu from hanging with blocking fsync
> >>>>   call - Stefan
> >>>> - Use virtio_st*_p() for endianess - Stefan
> >>>> - Correct indentation in qapi/misc.json - Eric
> >>>>
> >>>> Signed-off-by: Pankaj Gupta <pagupta@...hat.com>
> >>>> ---
> >>>>  hw/virtio/Makefile.objs                     |   3 +
> >>>>  hw/virtio/virtio-pci.c                      |  44 +++++
> >>>>  hw/virtio/virtio-pci.h                      |  14 ++
> >>>>  hw/virtio/virtio-pmem.c                     | 241
> >>>>  ++++++++++++++++++++++++++++
> >>>>  include/hw/pci/pci.h                        |   1 +
> >>>>  include/hw/virtio/virtio-pmem.h             |  42 +++++
> >>>>  include/standard-headers/linux/virtio_ids.h |   1 +
> >>>>  qapi/misc.json                              |  26 ++-
> >>>>  8 files changed, 371 insertions(+), 1 deletion(-)
> >>>>  create mode 100644 hw/virtio/virtio-pmem.c
> >>>>  create mode 100644 include/hw/virtio/virtio-pmem.h
> >>>>
> >>>> diff --git a/hw/virtio/Makefile.objs b/hw/virtio/Makefile.objs
> >>>> index 1b2799cfd8..7f914d45d0 100644
> >>>> --- a/hw/virtio/Makefile.objs
> >>>> +++ b/hw/virtio/Makefile.objs
> >>>> @@ -10,6 +10,9 @@ obj-$(CONFIG_VIRTIO_CRYPTO) += virtio-crypto.o
> >>>>  obj-$(call land,$(CONFIG_VIRTIO_CRYPTO),$(CONFIG_VIRTIO_PCI)) +=
> >>>>  virtio-crypto-pci.o
> >>>>  
> >>>>  obj-$(CONFIG_LINUX) += vhost.o vhost-backend.o vhost-user.o
> >>>> +ifeq ($(CONFIG_MEM_HOTPLUG),y)
> >>>> +obj-$(CONFIG_LINUX) += virtio-pmem.o
> >>>> +endif
> >>>>  obj-$(CONFIG_VHOST_VSOCK) += vhost-vsock.o
> >>>>  endif
> >>>>  
> >>>> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> >>>> index 3a01fe90f0..93d3fc05c7 100644
> >>>> --- a/hw/virtio/virtio-pci.c
> >>>> +++ b/hw/virtio/virtio-pci.c
> >>>> @@ -2521,6 +2521,49 @@ static const TypeInfo virtio_rng_pci_info = {
> >>>>      .class_init    = virtio_rng_pci_class_init,
> >>>>  };
> >>>>  
> >>>> +/* virtio-pmem-pci */
> >>>> +
> >>>> +static void virtio_pmem_pci_realize(VirtIOPCIProxy *vpci_dev, Error
> >>>> **errp)
> >>>> +{
> >>>> +    VirtIOPMEMPCI *vpmem = VIRTIO_PMEM_PCI(vpci_dev);
> >>>> +    DeviceState *vdev = DEVICE(&vpmem->vdev);
> >>>> +
> >>>> +    qdev_set_parent_bus(vdev, BUS(&vpci_dev->bus));
> >>>> +    object_property_set_bool(OBJECT(vdev), true, "realized", errp);
> >>>> +}
> >>>> +
> >>>> +static void virtio_pmem_pci_class_init(ObjectClass *klass, void *data)
> >>>> +{
> >>>> +    DeviceClass *dc = DEVICE_CLASS(klass);
> >>>> +    VirtioPCIClass *k = VIRTIO_PCI_CLASS(klass);
> >>>> +    PCIDeviceClass *pcidev_k = PCI_DEVICE_CLASS(klass);
> >>>> +    k->realize = virtio_pmem_pci_realize;
> >>>> +    set_bit(DEVICE_CATEGORY_MISC, dc->categories);
> >>>> +    pcidev_k->vendor_id = PCI_VENDOR_ID_REDHAT_QUMRANET;
> >>>> +    pcidev_k->device_id = PCI_DEVICE_ID_VIRTIO_PMEM;
> >>>> +    pcidev_k->revision = VIRTIO_PCI_ABI_VERSION;
> >>>> +    pcidev_k->class_id = PCI_CLASS_OTHERS;
> >>>> +}
> >>>> +
> >>>> +static void virtio_pmem_pci_instance_init(Object *obj)
> >>>> +{
> >>>> +    VirtIOPMEMPCI *dev = VIRTIO_PMEM_PCI(obj);
> >>>> +
> >>>> +    virtio_instance_init_common(obj, &dev->vdev, sizeof(dev->vdev),
> >>>> +                                TYPE_VIRTIO_PMEM);
> >>>> +    object_property_add_alias(obj, "memdev", OBJECT(&dev->vdev), "memdev",
> >>>> +                              &error_abort);
> >>>> +}
> >>>> +
> >>>> +static const TypeInfo virtio_pmem_pci_info = {
> >>>> +    .name          = TYPE_VIRTIO_PMEM_PCI,
> >>>> +    .parent        = TYPE_VIRTIO_PCI,
> >>>> +    .instance_size = sizeof(VirtIOPMEMPCI),
> >>>> +    .instance_init = virtio_pmem_pci_instance_init,
> >>>> +    .class_init    = virtio_pmem_pci_class_init,
> >>>> +};
> >>>> +
> >>>> +
> >>>>  /* virtio-input-pci */
> >>>>  
> >>>>  static Property virtio_input_pci_properties[] = {
> >>>> @@ -2714,6 +2757,7 @@ static void virtio_pci_register_types(void)
> >>>>      type_register_static(&virtio_balloon_pci_info);
> >>>>      type_register_static(&virtio_serial_pci_info);
> >>>>      type_register_static(&virtio_net_pci_info);
> >>>> +    type_register_static(&virtio_pmem_pci_info);
> >>>>  #ifdef CONFIG_VHOST_SCSI
> >>>>      type_register_static(&vhost_scsi_pci_info);
> >>>>  #endif
> >>>> diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
> >>>> index 813082b0d7..fe74fcad3f 100644
> >>>> --- a/hw/virtio/virtio-pci.h
> >>>> +++ b/hw/virtio/virtio-pci.h
> >>>> @@ -19,6 +19,7 @@
> >>>>  #include "hw/virtio/virtio-blk.h"
> >>>>  #include "hw/virtio/virtio-net.h"
> >>>>  #include "hw/virtio/virtio-rng.h"
> >>>> +#include "hw/virtio/virtio-pmem.h"
> >>>>  #include "hw/virtio/virtio-serial.h"
> >>>>  #include "hw/virtio/virtio-scsi.h"
> >>>>  #include "hw/virtio/virtio-balloon.h"
> >>>> @@ -57,6 +58,7 @@ typedef struct VirtIOInputHostPCI VirtIOInputHostPCI;
> >>>>  typedef struct VirtIOGPUPCI VirtIOGPUPCI;
> >>>>  typedef struct VHostVSockPCI VHostVSockPCI;
> >>>>  typedef struct VirtIOCryptoPCI VirtIOCryptoPCI;
> >>>> +typedef struct VirtIOPMEMPCI VirtIOPMEMPCI;
> >>>>  
> >>>>  /* virtio-pci-bus */
> >>>>  
> >>>> @@ -274,6 +276,18 @@ struct VirtIOBlkPCI {
> >>>>      VirtIOBlock vdev;
> >>>>  };
> >>>>  
> >>>> +/*
> >>>> + * virtio-pmem-pci: This extends VirtioPCIProxy.
> >>>> + */
> >>>> +#define TYPE_VIRTIO_PMEM_PCI "virtio-pmem-pci"
> >>>> +#define VIRTIO_PMEM_PCI(obj) \
> >>>> +        OBJECT_CHECK(VirtIOPMEMPCI, (obj), TYPE_VIRTIO_PMEM_PCI)
> >>>> +
> >>>> +struct VirtIOPMEMPCI {
> >>>> +    VirtIOPCIProxy parent_obj;
> >>>> +    VirtIOPMEM vdev;
> >>>> +};
> >>>> +
> >>>>  /*
> >>>>   * virtio-balloon-pci: This extends VirtioPCIProxy.
> >>>>   */
> >>>> diff --git a/hw/virtio/virtio-pmem.c b/hw/virtio/virtio-pmem.c
> >>>> new file mode 100644
> >>>> index 0000000000..08c96d7e80
> >>>> --- /dev/null
> >>>> +++ b/hw/virtio/virtio-pmem.c
> >>>> @@ -0,0 +1,241 @@
> >>>> +/*
> >>>> + * Virtio pmem device
> >>>> + *
> >>>> + * Copyright (C) 2018 Red Hat, Inc.
> >>>> + * Copyright (C) 2018 Pankaj Gupta <pagupta@...hat.com>
> >>>> + *
> >>>> + * This work is licensed under the terms of the GNU GPL, version 2.
> >>>> + * See the COPYING file in the top-level directory.
> >>>> + *
> >>>> + */
> >>>> +
> >>>> +#include "qemu/osdep.h"
> >>>> +#include "qapi/error.h"
> >>>> +#include "qemu-common.h"
> >>>> +#include "qemu/error-report.h"
> >>>> +#include "hw/virtio/virtio-access.h"
> >>>> +#include "hw/virtio/virtio-pmem.h"
> >>>> +#include "hw/mem/memory-device.h"
> >>>> +#include "block/aio.h"
> >>>> +#include "block/thread-pool.h"
> >>>> +
> >>>> +typedef struct VirtIOPMEMresp {
> >>>> +    int ret;
> >>>> +} VirtIOPMEMResp;
> >>>> +
> >>>> +typedef struct VirtIODeviceRequest {
> >>>> +    VirtQueueElement elem;
> >>>> +    int fd;
> >>>> +    VirtIOPMEM *pmem;
> >>>> +    VirtIOPMEMResp resp;
> >>>> +} VirtIODeviceRequest;
> >>>> +
> >>>> +static int worker_cb(void *opaque)
> >>>> +{
> >>>> +    VirtIODeviceRequest *req = opaque;
> >>>> +    int err = 0;
> >>>> +
> >>>> +    /* flush raw backing image */
> >>>> +    err = fsync(req->fd);
> >>>> +    if (err != 0) {
> >>>> +        err = errno;
> >>>> +    }
> >>>> +    req->resp.ret = err;  
> >>>
> >>> Host question: are you returning the guest errno code to the host?  
> >>
> >> No. I am returning error code from the host in-case of host fsync
> >> failure, otherwise returning zero.  
> > 
> > I think that's what Luiz meant.  errno constants are not portable
> > between operating systems and architectures.  Therefore they cannot be
> > used in external interfaces in software that expects to communicate with
> > other systems.
> > 
> > It will be necessary to define specific constants for virtio-pmem
> > instead of passing errno from the host to guest.
> >   
> 
> In general, I wonder if we should report errors at all or rather *kill*
> the guest. That might sound harsh, but think about the following scenario:

I almost sure that I read in the nvdimm spec that real hardware will
cause a memory error on sync or read/write error. If we truly want
to emulate this, then I guess QEMU should be able to inject a memory
error for the entire region instead of returning the fsync() error
to the guest.

> fsync() fails due to some block that cannot e.g. be written (e.g.
> network connection failed). What happens if our guest tries to
> read/write that mmaped block? (e.g. network connection failed).

I think it gets a SIGBUS? Btw, I think that QEMU already has the
machinery to turn a SIGBUS into an memory error in the guest.

> I assume we'll get a signal an get killed? So we are trying to optimize
> one special case (fsync()) although every read/write is prone to kill
> the guest. And as soon as the guest will try to access the block that
> made fsync fail, we will crash the guest either way.

I think you have a point.

> 
> I assume the main problem is that we are trying to take a file (with all
> the errors that can happen during read/write/fsync) and make it look
> like memory (dax). On ordinary block access, we can forward errors, but
> not if it's memory (maybe using MCE, but it's complicated and
> architecture specific).
> 
> So I wonder if we should rather assume that our backend file is placed
> on some stable storage that cannot easily fail.
> 
> (we might have the same problem with NVDIMM right now, at least the
> memory reading/writing part)
> 
> It's complicated and I am not a block level expert :)
> 
> > Stefan
> >   
> 
> 

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