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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Jag Raman (jag.raman@oracle.com) wrote:
> 
> 
> On 3/5/2020 7:31 AM, Dr. David Alan Gilbert wrote:
> > * Jagannathan Raman (jag.raman@oracle.com) 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>
> > > ---
> > >   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.
> 
> Hi Dave,
> 
> Upon closer inspection, we found that the migration code on the remote
> (which uses QEMUFile) could sometimes set an error on the channel using
> qemu_file_set_error(). We needed to use qemu_file_get_error() to catch
> these errors and abort migration. So we had to stick with QEMUFile at
> the receiving end as well.

I realise you need to use a QEMUFile on the part that connects to the
Savevm code/vmstate code - but that doesn't mean you need it on the side
that just connects between your pipe and the qemu.

> > 
> > > +    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?
> 
> Yes, Dave. That is correct.
> 
> > 
> > > +    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.
> 
> Sure, will do.
> 
> > 
> > > +    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).
> 
> We noticed that VMSTATE_BUFFER requires that the buffer be part of a
> "struct" and that the buffer is an array. Since the buffer we're using
> is neither an array nor part of a "struct", we decided to go with
> writing the buffer directly to the migration stream in proxy_post_save()
> instead of using VMSTATE_BUFFER.
> 
> I think we should nevertheless g_free this buffer in post_save like you
> pointed out.

Note I'm suggesting using VMSTATE_VBUFFER, not VMSTATE_BUFFER;
VBUFFER is expecting a unsigned char *; it does expect that to be in
your structure, so I'd expect your proxy to have a mig_data and uint32_t
mig_len fields.

Dave

> Thank you!
> --
> Jag
> 
> > 
> > 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 / dgilbert@redhat.com / Manchester, UK
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK