On Wed, Apr 01, 2026 at 03:27:15PM +0200, Peter Krempa via Devel wrote:
> From: Peter Krempa <[email protected]>
>
> In certain cases it's useful for the caller to be able to determine if
> given API supports some flags.
>
> To do this with 'virDomainBlockResize', if
> 'VIR_DOMAIN_BLOCK_RESIZE_PROBE_FLAGS' is specified, the API will return
> success right after flag verification, regardless of other input
> arguments and without modifying anything.
>
> Signed-off-by: Peter Krempa <[email protected]>
> ---
> include/libvirt/libvirt-domain.h | 9 +++++++++
> src/libvirt-domain.c | 12 ++++++++++++
> src/qemu/qemu_driver.c | 7 ++++++-
> src/vz/vz_driver.c | 7 ++++++-
> 4 files changed, 33 insertions(+), 2 deletions(-)
>
> diff --git a/include/libvirt/libvirt-domain.h
> b/include/libvirt/libvirt-domain.h
> index 4a8e3114b3..113c7eefe6 100644
> --- a/include/libvirt/libvirt-domain.h
> +++ b/include/libvirt/libvirt-domain.h
> @@ -2354,6 +2354,15 @@ typedef enum {
>
> /* Disallow shrinking (Since: 12.3.0) */
> VIR_DOMAIN_BLOCK_RESIZE_EXTEND = 1 << 2,
> +
> + /* Ensures that the 'virDomainBlockResize' API returns success after
> + * checking support of 'flags' regardless of other arguments without
> + * actually modifying any aspect of the domain.
> + *
> + * Use of the flag thus allows probing for support of other flags of this
> + * API. (Since: 12.3.0) */
> + VIR_DOMAIN_BLOCK_RESIZE_PROBE_FLAGS = 1 << 3,
> +
> } virDomainBlockResizeFlags;
>
> int virDomainBlockResize (virDomainPtr dom,
> diff --git a/src/libvirt-domain.c b/src/libvirt-domain.c
> index db9eea5774..1b49d2f7e5 100644
> --- a/src/libvirt-domain.c
> +++ b/src/libvirt-domain.c
> @@ -6469,6 +6469,18 @@ virDomainBlockPeek(virDomainPtr dom,
> * can be found by calling virDomainGetXMLDesc() and inspecting
> * elements within //domain/devices/disk.
> *
> + * If @flags contains VIR_DOMAIN_BLOCK_RESIZE_PROBE_FLAGS (since 12.3.0) the
> + * API will return success right after checking flags for support without
> + * modifying the disk in any way, thus allowing probing of flags with the
> + * following algorithm
> + *
> + * supp = virDomainBlockResize(dom, "", 0,
> VIR_DOMAIN_BLOCK_RESIZE_PROBE_FLAGS | VIR_DOMAIN_BLOCK_RESIZE_EXTEND);
> + *
> + * if (supp == 0)
> + * //flag supported
> + * else
> + * virResetLastError();
This suggests "flag" (singular), but if you pass many flags at once,
then the error only tells you that at least 1 flag is invalid, but
not which flag is invalid. So we need O(n) calls with PROBE_FLAGS
if we want to check n different flags as distinct things.
Also the "else" branch can also still be an error for non-probe
related issues. eg there are many errors that the remote driver
can raise from a transport POV.
How much better is this than simply trying the RESIZE_EXTEND usage
unconditionally, and then falling back to a different code path if
seeing INVALID_ARG error ?
> + *
> * Note that this call may fail if the underlying virtualization hypervisor
> * does not support it; this call requires privileged access to the
> * hypervisor.
> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
> index 551553df14..aa064ca7a9 100644
> --- a/src/qemu/qemu_driver.c
> +++ b/src/qemu/qemu_driver.c
> @@ -9449,7 +9449,12 @@ qemuDomainBlockResize(virDomainPtr dom,
>
> virCheckFlags(VIR_DOMAIN_BLOCK_RESIZE_BYTES |
> VIR_DOMAIN_BLOCK_RESIZE_CAPACITY |
> - VIR_DOMAIN_BLOCK_RESIZE_EXTEND, -1);
> + VIR_DOMAIN_BLOCK_RESIZE_EXTEND |
> + VIR_DOMAIN_BLOCK_RESIZE_PROBE_FLAGS, -1);
> +
> + /* Assume success if flag probing is requested */
> + if (flags & VIR_DOMAIN_BLOCK_RESIZE_PROBE_FLAGS)
> + return 0;
>
> /* We prefer operating on bytes. */
> if ((flags & VIR_DOMAIN_BLOCK_RESIZE_BYTES) == 0) {
> diff --git a/src/vz/vz_driver.c b/src/vz/vz_driver.c
> index 2d8878fe7f..efb0144093 100644
> --- a/src/vz/vz_driver.c
> +++ b/src/vz/vz_driver.c
> @@ -3901,7 +3901,12 @@ vzDomainBlockResize(virDomainPtr domain,
> int ret = -1;
> bool job = false;
>
> - virCheckFlags(VIR_DOMAIN_BLOCK_RESIZE_BYTES, -1);
> + virCheckFlags(VIR_DOMAIN_BLOCK_RESIZE_BYTES |
> + VIR_DOMAIN_BLOCK_RESIZE_PROBE_FLAGS, -1);
> +
> + /* Assume success if flag probing is requested */
> + if (flags & VIR_DOMAIN_BLOCK_RESIZE_PROBE_FLAGS)
> + return 0;
>
> if (!(dom = vzDomObjFromDomain(domain)))
> goto cleanup;
> --
> 2.53.0
>
With regards,
Daniel
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