Re: [RFC v4 PATCH 41/49] multi-process/mig: Enable VMSD save in the Proxy object

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Oct 24, 2019 at 05:09:22AM -0400, Jagannathan Raman 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 <elena.ufimtseva@oracle.com>
> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
> ---
>  New patch in v4
> 
>  hw/proxy/qemu-proxy.c         | 132 ++++++++++++++++++++++++++++++++++++++++++
>  include/hw/proxy/qemu-proxy.h |   2 +
>  include/io/mpqemu-link.h      |   1 +
>  3 files changed, 135 insertions(+)
> 
> diff --git a/hw/proxy/qemu-proxy.c b/hw/proxy/qemu-proxy.c
> index 623a6c5..ce72e6a 100644
> --- a/hw/proxy/qemu-proxy.c
> +++ b/hw/proxy/qemu-proxy.c
> @@ -52,6 +52,14 @@
>  #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);
> @@ -62,6 +70,9 @@ static void stop_heartbeat_timer(void);
>  static void childsig_handler(int sig, siginfo_t *siginfo, void *ctx);
>  static void broadcast_msg(MPQemuMsg *msg, bool need_reply);
>  
> +#define PAGE_SIZE getpagesize()
> +uint8_t *mig_data;
> +
>  static void childsig_handler(int sig, siginfo_t *siginfo, void *ctx)
>  {
>      /* TODO: Add proper handler. */
> @@ -357,14 +368,135 @@ 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);

There is a pretty large set of APIs hiding behind the qemu_get_byte
call, which does not give me confidence that...

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

....this is a safe way to stop the thread executing without
resulting in memory being leaked.

In addition thread cancellation is asynchronous, so the thread
may still be using the QEMUFile object while....

> +    qemu_fclose(f_remote);

..this is closing it. This feels like it is a crash danger.


> +    close(fd[1]);
> +
> +    return 0;
> +}

Regards,
Daniel
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