Re: [PATCH v5 41/50] multi-process/mig: Enable VMSD save in the Proxy object

[Dr. David Alan Gilbert]

* Jagannathan Raman (address@hidden) wrote:
> Collect the VMSD from remote process on the source and save
> it to the channel leading to the destination
> 
> Signed-off-by: Elena Ufimtseva <address@hidden>
> Signed-off-by: John G Johnson <address@hidden>
> Signed-off-by: Jagannathan Raman <address@hidden>
> ---
>  v4 -> v5:
>   - Using qemu_file_shutdown() instead of qemu_thread_cancel(). Removed patch
>     which introduced qemu_thread_cancel()
> 
>  hw/proxy/qemu-proxy.c         | 135 
> ++++++++++++++++++++++++++++++++++++++++++
>  include/hw/proxy/qemu-proxy.h |   2 +
>  include/io/mpqemu-link.h      |   1 +
>  3 files changed, 138 insertions(+)
> 
> diff --git a/hw/proxy/qemu-proxy.c b/hw/proxy/qemu-proxy.c
> index b1b9282..19f0dbb 100644
> --- a/hw/proxy/qemu-proxy.c
> +++ b/hw/proxy/qemu-proxy.c
> @@ -23,6 +23,14 @@
>  #include "util/event_notifier-posix.c"
>  #include "hw/boards.h"
>  #include "include/qemu/log.h"
> +#include "io/channel.h"
> +#include "migration/qemu-file-types.h"
> +#include "qapi/error.h"
> +#include "io/channel-util.h"
> +#include "migration/qemu-file-channel.h"
> +#include "migration/qemu-file.h"
> +#include "migration/migration.h"
> +#include "migration/vmstate.h"
>  
>  QEMUTimer *hb_timer;
>  static void pci_proxy_dev_realize(PCIDevice *dev, Error **errp);
> @@ -35,6 +43,9 @@ static void broadcast_init(void);
>  static int config_op_send(PCIProxyDev *dev, uint32_t addr, uint32_t *val, 
> int l,
>                            unsigned int op);
>  
> +#define PAGE_SIZE qemu_real_host_page_size
> +uint8_t *mig_data;
> +
>  static void childsig_handler(int sig, siginfo_t *siginfo, void *ctx)
>  {
>      /* TODO: Add proper handler. */
> @@ -460,6 +471,129 @@ static void pci_proxy_dev_inst_init(Object *obj)
>      dev->mem_init = false;
>  }
>  
> +typedef struct {
> +    QEMUFile *rem;
> +    PCIProxyDev *dev;
> +} proxy_mig_data;
> +
> +static void *proxy_mig_out(void *opaque)
> +{
> +    proxy_mig_data *data = opaque;
> +    PCIProxyDev *dev = data->dev;
> +    uint8_t byte;
> +    uint64_t data_size = PAGE_SIZE;
> +
> +    mig_data = g_malloc(data_size);
> +
> +    while (true) {
> +        byte = qemu_get_byte(data->rem);
> +
> +        if (qemu_file_get_error(data->rem)) {
> +            break;
> +        }
> +
> +        mig_data[dev->migsize++] = byte;
> +        if (dev->migsize == data_size) {
> +            data_size += PAGE_SIZE;
> +            mig_data = g_realloc(mig_data, data_size);
> +        }
> +    }
> +
> +    return NULL;
> +}
> +
> +static int proxy_pre_save(void *opaque)
> +{
> +    PCIProxyDev *pdev = opaque;
> +    proxy_mig_data *mig_data;
> +    QEMUFile *f_remote;
> +    MPQemuMsg msg = {0};
> +    QemuThread thread;
> +    Error *err = NULL;
> +    QIOChannel *ioc;
> +    uint64_t size;
> +    int fd[2];
> +
> +    if (socketpair(AF_UNIX, SOCK_STREAM, 0, fd)) {
> +        return -1;
> +    }
> +
> +    ioc = qio_channel_new_fd(fd[0], &err);
> +    if (err) {
> +        error_report_err(err);
> +        return -1;
> +    }
> +
> +    qio_channel_set_name(QIO_CHANNEL(ioc), "PCIProxyDevice-mig");
> +
> +    f_remote = qemu_fopen_channel_input(ioc);
> +
> +    pdev->migsize = 0;
> +
> +    mig_data = g_malloc0(sizeof(proxy_mig_data));
> +    mig_data->rem = f_remote;

This feels way too complicated.   Since we know f_remote is always just
a simple fd we're getting we don't need to use qio or qemu_file here;
just use the fd - nice and simple.

Then the code to read it can just use read(2) with a sensible size chunk
rather than reading a byte at a time.

> +    mig_data->dev = pdev;
> +
> +    qemu_thread_create(&thread, "Proxy MIG_OUT", proxy_mig_out, mig_data,
> +                       QEMU_THREAD_DETACHED);

I'm just checking why a thread is necessary; is this because you need to
be able to start reading before you block waiting for the remote to tell
you the size - worrying that if you don't start reading then the remote
might block waiting for us?

> +    msg.cmd = START_MIG_OUT;
> +    msg.bytestream = 0;
> +    msg.num_fds = 2;
> +    msg.fds[0] = fd[1];
> +    msg.fds[1] = GET_REMOTE_WAIT;
> +
> +    mpqemu_msg_send(&msg, pdev->mpqemu_link->com);
> +    size = wait_for_remote(msg.fds[1]);
> +    PUT_REMOTE_WAIT(msg.fds[1]);
> +
> +    assert(size != ULLONG_MAX);
> +
> +    /*
> +     * migsize is being update by a separate thread. Using volatile to
> +     * instruct the compiler to fetch the value of this variable from
> +     * memory during every read
> +     */
> +    while (*((volatile uint64_t *)&pdev->migsize) < size) {
> +    }

Hmm.  I suggest the following:

  a) You create a shared 'size' variable;  and initialize it to
    UINT64_MAX.
  b) The thread uses atomic_read(shared_size) to read that value.
  c) When you receive the size from the remote you do
     atomic_set(&shared_size, size);
  d) The thread does:
     while (received_size < atomic_read(&shared_size))

     so the thread will quit either on EOF or it hitting the size.

  e) We pthread_join here to wait for the thread
  f) We then check a received size to make sure it matches what we
expect.

That removes the tight loop.

> +    qemu_file_shutdown(f_remote);
> +
> +    qemu_fclose(f_remote);
> +    close(fd[1]);
> +
> +    return 0;
> +}
> +
> +static int proxy_post_save(void *opaque)
> +{
> +    MigrationState *ms = migrate_get_current();
> +    PCIProxyDev *pdev = opaque;
> +    uint64_t pos = 0;
> +
> +    while (pos < pdev->migsize) {
> +        qemu_put_byte(ms->to_dst_file, mig_data[pos]);
> +        pos++;
> +    }
> +
> +    qemu_fflush(ms->to_dst_file);
> +
> +    return 0;

I don't think you need that.

> +}
> +
> +const VMStateDescription vmstate_pci_proxy_device = {
> +    .name = "PCIProxyDevice",
> +    .version_id = 2,
> +    .minimum_version_id = 1,
> +    .pre_save = proxy_pre_save,
> +    .post_save = proxy_post_save,
> +    .fields = (VMStateField[]) {
> +        VMSTATE_PCI_DEVICE(parent_dev, PCIProxyDev),
> +        VMSTATE_UINT64(migsize, PCIProxyDev),

I think instead you should use a VMSTATE_VBUFFER here to save
the mig_data.
What we should do is the post_save should g_free the buffer.
(mig_data should be a field in proxy).

Dave


> +        VMSTATE_END_OF_LIST()
> +    }
> +};
> +
>  static void pci_proxy_dev_class_init(ObjectClass *klass, void *data)
>  {
>      PCIDeviceClass *k = PCI_DEVICE_CLASS(klass);
> @@ -471,6 +605,7 @@ static void pci_proxy_dev_class_init(ObjectClass *klass, 
> void *data)
>      k->config_write = pci_proxy_write_config;
>  
>      dc->reset = proxy_device_reset;
> +    dc->vmsd = &vmstate_pci_proxy_device;
>  }
>  
>  static const TypeInfo pci_proxy_dev_type_info = {
> diff --git a/include/hw/proxy/qemu-proxy.h b/include/hw/proxy/qemu-proxy.h
> index 5de8129..537c227 100644
> --- a/include/hw/proxy/qemu-proxy.h
> +++ b/include/hw/proxy/qemu-proxy.h
> @@ -75,6 +75,8 @@ struct PCIProxyDev {
>                          bool need_spawn, Error **errp);
>  
>      ProxyMemoryRegion region[PCI_NUM_REGIONS];
> +
> +    uint64_t migsize;
>  };
>  
>  typedef struct PCIProxyDevClass {
> diff --git a/include/io/mpqemu-link.h b/include/io/mpqemu-link.h
> index d2234ca..b42c003 100644
> --- a/include/io/mpqemu-link.h
> +++ b/include/io/mpqemu-link.h
> @@ -63,6 +63,7 @@ typedef enum {
>      PROXY_PING,
>      MMIO_RETURN,
>      DEVICE_RESET,
> +    START_MIG_OUT,
>      MAX,
>  } mpqemu_cmd_t;
>  
> -- 
> 1.8.3.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK