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Re: [libvirt] [PATCH] qemu: call drive_unplug in DetachPciDiskDevice



On Thu, Oct 21, 2010 at 08:50:35AM -0500, Ryan Harper wrote:
> Currently libvirt doesn't confirm whether the guest has responded to the
> disk removal request.  In some cases this can leave the guest with
> continued access to the device while the mgmt layer believes that it has
> been removed.  With a recent qemu monitor command[1] we can
> deterministically revoke a guests access to the disk (on the QEMU side)
> to ensure no futher access is permitted.
> 
> This patch adds support for the drive_unplug() command and introduces it
> in the disk removal paths.  There is some discussion to be had about how
> to handle the case where the guest is running in a QEMU without this
> command (and the fact that we currently don't have a way of detecting
> what monitor commands are available).

Basically we try to run the command and then catch the failure.

For QMP, we can check for a error with a class of 'CommandNotFound',

For HMP, QEMU will print 'unknown command' in the reply.

Neither is ideal, since neither is a guarenteed part of the monitor
interface, but it is all we have to go on, and ensure other critical
errors will still be treated as fatal by libvirt.

> My current implementation assumes that if you don't have a QEMU with
> this capability that we should fail the device removal.  This is a
> strong statement around hotplug that isn't consistent with previous
> releases so I'm open to other approachs, but given the potential data
> leakage problem hot-remove can lead to without drive_unplug, I think
> it's the right thing to do.

I don't think we can do this, since it obviously breaks every single
existing deployment out there. Users who have sVirt enabled will
have a level of protection from the data leakage, so I don't think
it is a severe problem. 

> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> index abd8e9d..c7f4746 100644
> --- a/src/qemu/qemu_driver.c
> +++ b/src/qemu/qemu_driver.c
> @@ -8646,6 +8646,7 @@ static int qemudDomainDetachPciDiskDevice(struct qemud_driver *driver,
>      virDomainDiskDefPtr detach = NULL;
>      qemuDomainObjPrivatePtr priv = vm->privateData;
>      virCgroupPtr cgroup = NULL;
> +    char drivestr[PATH_MAX];
>  
>      i = qemudFindDisk(vm->def, dev->data.disk->dst);
>  
> @@ -8673,13 +8674,34 @@ static int qemudDomainDetachPciDiskDevice(struct qemud_driver *driver,
>          goto cleanup;
>      }
>  
> +    /* build the actual drive id string as the disk->info.alias doesn't
> +     * contain the QEMU_DRIVE_HOST_PREFIX that is passed to qemu */
> +    if ((ret = snprintf(drivestr, sizeof(drivestr), "%s%s",
> +                   QEMU_DRIVE_HOST_PREFIX,
> +                   detach->info.alias))
> +        < 0 || ret >= sizeof(drivestr)) {
> +        virReportOOMError();
> +        goto cleanup;
> +    }
> +
>      qemuDomainObjEnterMonitorWithDriver(driver, vm);
>      if (qemuCmdFlags & QEMUD_CMD_FLAG_DEVICE) {
> +        ret = qemuMonitorDriveUnplug(priv->mon, drivestr);
> +        DEBUG("DriveUnplug ret=%d", ret);
> +        if (ret != 0) {
> +            qemuDomainObjExitMonitor(vm);
> +            goto cleanup;
> +        }
>          if (qemuMonitorDelDevice(priv->mon, detach->info.alias) < 0) {
>              qemuDomainObjExitMonitor(vm);
>              goto cleanup;
>          }
>      } else {
> +        ret = qemuMonitorDriveUnplug(priv->mon, drivestr);
> +        if (ret != 0) {
> +            qemuDomainObjExitMonitor(vm);
> +            goto cleanup;
> +        }
>          if (qemuMonitorRemovePCIDevice(priv->mon,
>                                         &detach->info.addr.pci) < 0) {
>              qemuDomainObjExitMonitor(vm);
> @@ -8723,6 +8745,7 @@ static int qemudDomainDetachSCSIDiskDevice(struct qemud_driver *driver,
>      virDomainDiskDefPtr detach = NULL;
>      qemuDomainObjPrivatePtr priv = vm->privateData;
>      virCgroupPtr cgroup = NULL;
> +    char drivestr[PATH_MAX];
>  
>      i = qemudFindDisk(vm->def, dev->data.disk->dst);
>  
> @@ -8749,7 +8772,21 @@ static int qemudDomainDetachSCSIDiskDevice(struct qemud_driver *driver,
>          }
>      }
>  
> +    /* build the actual drive id string as the disk->info.alias doesn't
> +     * contain the QEMU_DRIVE_HOST_PREFIX that is passed to qemu */
> +    if ((ret = snprintf(drivestr, sizeof(drivestr), "%s%s",
> +                   QEMU_DRIVE_HOST_PREFIX,
> +                   detach->info.alias))
> +        < 0 || ret >= sizeof(drivestr)) {
> +        virReportOOMError();
> +        goto cleanup;
> +    }
> +
>      qemuDomainObjEnterMonitorWithDriver(driver, vm);
> +    if (qemuMonitorDriveUnplug(priv->mon, drivestr) < 0) {
> +        qemuDomainObjExitMonitor(vm);
> +        goto cleanup;
> +    }
>      if (qemuMonitorDelDevice(priv->mon, detach->info.alias) < 0) {
>          qemuDomainObjExitMonitor(vm);
>          goto cleanup;
> diff --git a/src/qemu/qemu_monitor.c b/src/qemu/qemu_monitor.c
> index 2366fdb..285381d 100644
> --- a/src/qemu/qemu_monitor.c
> +++ b/src/qemu/qemu_monitor.c
> @@ -1781,6 +1781,25 @@ int qemuMonitorGetAllPCIAddresses(qemuMonitorPtr mon,
>      return ret;
>  }
>  
> +int qemuMonitorDriveUnplug(qemuMonitorPtr mon,
> +                         const char *drivestr)
> +{
> +    DEBUG("mon=%p drivestr=%s", mon, drivestr);
> +    int ret;
> +
> +    if (!mon) {
> +        qemuReportError(VIR_ERR_INVALID_ARG, "%s",
> +                        _("monitor must not be NULL"));
> +        return -1;
> +    }
> +
> +    if (mon->json)
> +        ret = qemuMonitorJSONDriveUnplug(mon, drivestr);
> +    else
> +        ret = qemuMonitorTextDriveUnplug(mon, drivestr);
> +    return ret;
> +}
> +
>  int qemuMonitorDelDevice(qemuMonitorPtr mon,
>                           const char *devalias)
>  {
> diff --git a/src/qemu/qemu_monitor.h b/src/qemu/qemu_monitor.h
> index 48f4c20..bfe3641 100644
> --- a/src/qemu/qemu_monitor.h
> +++ b/src/qemu/qemu_monitor.h
> @@ -381,6 +381,9 @@ int qemuMonitorDelDevice(qemuMonitorPtr mon,
>  int qemuMonitorAddDrive(qemuMonitorPtr mon,
>                          const char *drivestr);
>  
> +int qemuMonitorDriveUnplug(qemuMonitorPtr mon,
> +                        const char *drivestr);
> +
>  int qemuMonitorSetDrivePassphrase(qemuMonitorPtr mon,
>                                    const char *alias,
>                                    const char *passphrase);
> diff --git a/src/qemu/qemu_monitor_json.c b/src/qemu/qemu_monitor_json.c
> index d3ab25f..e99adac 100644
> --- a/src/qemu/qemu_monitor_json.c
> +++ b/src/qemu/qemu_monitor_json.c
> @@ -2243,6 +2243,30 @@ int qemuMonitorJSONAddDrive(qemuMonitorPtr mon,
>  }
>  
>  
> +int qemuMonitorJSONDriveUnplug(qemuMonitorPtr mon,
> +                             const char *drivestr)
> +{
> +    int ret;
> +    virJSONValuePtr cmd;
> +    virJSONValuePtr reply = NULL;
> +
> +    DEBUG("JSONDriveUnplug drivestr=%s", drivestr);
> +    cmd = qemuMonitorJSONMakeCommand("drive_unplug",
> +                                     "s:id", drivestr,
> +                                     NULL);
> +    if (!cmd)
> +        return -1;
> +
> +    ret = qemuMonitorJSONCommand(mon, cmd, &reply);
> +
> +    if (ret == 0)
> +        ret = qemuMonitorJSONCheckError(cmd, reply);
> +
> +    virJSONValueFree(cmd);
> +    virJSONValueFree(reply);
> +    return ret;
> +}
> +
>  int qemuMonitorJSONSetDrivePassphrase(qemuMonitorPtr mon,
>                                        const char *alias,
>                                        const char *passphrase)
> diff --git a/src/qemu/qemu_monitor_json.h b/src/qemu/qemu_monitor_json.h
> index 94806c1..6a8692e 100644
> --- a/src/qemu/qemu_monitor_json.h
> +++ b/src/qemu/qemu_monitor_json.h
> @@ -188,6 +188,9 @@ int qemuMonitorJSONDelDevice(qemuMonitorPtr mon,
>  int qemuMonitorJSONAddDrive(qemuMonitorPtr mon,
>                              const char *drivestr);
>  
> +int qemuMonitorJSONDriveUnplug(qemuMonitorPtr mon,
> +                            const char *drivestr);
> +
>  int qemuMonitorJSONSetDrivePassphrase(qemuMonitorPtr mon,
>                                        const char *alias,
>                                        const char *passphrase);
> diff --git a/src/qemu/qemu_monitor_text.c b/src/qemu/qemu_monitor_text.c
> index 69971a6..ded3078 100644
> --- a/src/qemu/qemu_monitor_text.c
> +++ b/src/qemu/qemu_monitor_text.c
> @@ -2380,6 +2380,45 @@ cleanup:
>      return ret;
>  }
>  
> +int qemuMonitorTextDriveUnplug(qemuMonitorPtr mon,
> +                             const char *drivestr)
> +{
> +    char *cmd = NULL;
> +    char *reply = NULL;
> +    char *safedev;
> +    int ret = -1;
> +    DEBUG("TextDriveUnplug drivestr=%s", drivestr);
> +
> +    if (!(safedev = qemuMonitorEscapeArg(drivestr))) {
> +        virReportOOMError();
> +        goto cleanup;
> +    }
> +
> +    if (virAsprintf(&cmd, "drive_unplug %s", safedev) < 0) {
> +        virReportOOMError();
> +        goto cleanup;
> +    }
> +
> +    if (qemuMonitorCommand(mon, cmd, &reply) < 0) {
> +        qemuReportError(VIR_ERR_OPERATION_FAILED,
> +                        _("cannot unplug %s drive"), drivestr);
> +        goto cleanup;
> +    }
> +
> +    if (STRNEQ(reply, "")) {
> +        qemuReportError(VIR_ERR_OPERATION_FAILED,
> +                        _("unplugging %s drive failed: %s"), drivestr, reply);
> +        goto cleanup;
> +    }
> +
> +    ret = 0;
> +
> +cleanup:
> +    VIR_FREE(cmd);
> +    VIR_FREE(reply);
> +    VIR_FREE(safedev);
> +    return ret;
> +}
>  
>  int qemuMonitorTextSetDrivePassphrase(qemuMonitorPtr mon,
>                                        const char *alias,
> diff --git a/src/qemu/qemu_monitor_text.h b/src/qemu/qemu_monitor_text.h
> index c017509..8355ce8 100644
> --- a/src/qemu/qemu_monitor_text.h
> +++ b/src/qemu/qemu_monitor_text.h
> @@ -186,6 +186,9 @@ int qemuMonitorTextDelDevice(qemuMonitorPtr mon,
>  int qemuMonitorTextAddDrive(qemuMonitorPtr mon,
>                               const char *drivestr);
>  
> +int qemuMonitorTextDriveUnplug(qemuMonitorPtr mon,
> +                             const char *drivestr);
> +
>  int qemuMonitorTextSetDrivePassphrase(qemuMonitorPtr mon,
>                                        const char *alias,
>                                        const char *passphrase);


The patch all looks good to me, apart from issues around handling the
command-not-found case as non-fatal.


Regards,
Daniel
-- 
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