[libvirt] [PATCH v1 1/3] adding unplug_timeout QEMU conf

[Ján Tomko]

On Sun, Aug 18, 2019 at 06:45:29PM -0300, Daniel Henrique Barboza wrote:
>For some architectures and setups, device removal can take
>longer than the default 5 seconds. This results in commands
>such as 'virsh setvcpus' to fire timeout messages even if
>the actual operation happened in the guest, causing confusion
>for the user.
>

The commit that introduced this error message:
    commit e3229f6e4461cd1721dc68a32e16ab1718ae716e
        qemu: hotplug: Add support for VCPU unplug

specifically says that we treat this differently than regular device
detach:

    As the new code is using device_del all the implications of using it
    are present. Contrary to the device deletion code, the vcpu deletion
    code fails if the unplug request is not executed in time.

Technically, we already did execute the unplug request so we lie to the
user saying "operation failed".

Maybe we can revisit the decision? [cc-ing pkrempa who added this]

>This patch adds a new qemu.conf parameter called 'unplug_timeout'
>to handle these cases. If left unset, the current default
>timeout is used. To avoid user 'experimentation' with small
>timeouts, the current timeout is also the minimal value
>allowed.
>

The reason for this timeout is that we originally promised something
that we cannot deliver - a synchronous device detach API, while the
operation itself is asynchronous. I'm not a fan of exposing it and
making it configurable.

>Signed-off-by: Daniel Henrique Barboza <danielhb413 at gmail.com>
>---
> src/qemu/libvirtd_qemu.aug         |  3 +++
> src/qemu/qemu.conf                 |  4 ++++
> src/qemu/qemu_conf.c               | 26 ++++++++++++++++++++++++++
> src/qemu/qemu_conf.h               |  5 +++++
> src/qemu/qemu_driver.c             |  2 ++
> src/qemu/test_libvirtd_qemu.aug.in |  1 +
> 6 files changed, 41 insertions(+)
>

[...]

>diff --git a/src/qemu/qemu_conf.h b/src/qemu/qemu_conf.h
>index 0cbddd7a9c..29824e4e35 100644
>--- a/src/qemu/qemu_conf.h
>+++ b/src/qemu/qemu_conf.h
>@@ -214,6 +214,8 @@ struct _virQEMUDriverConfig {
>     gid_t swtpm_group;
>
>     char **capabilityfilters;
>+
>+    unsigned int unplugTimeout;
> };
>
> /* Main driver state */
>@@ -294,6 +296,9 @@ struct _virQEMUDriver {
>
>     /* Immutable pointer, self-locking APIs */
>     virHashAtomicPtr migrationErrors;
>+
>+    /* Immutable value */
>+    unsigned int unplugTimeout;
> };
>

Why store this value twice?

Jano
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