[PATCH 0/2] qemu: Fix error reporting when QEMU capability probing fails
See both patches for explanation and examples. Peter Krempa (2): qemuProcessQMPStop: Preserve error when killing qemu used for capability probing virQEMUCapsLogProbeFailure: Reraise error with more information src/qemu/qemu_capabilities.c | 4 ++++ src/qemu/qemu_process.c | 4 ++++ 2 files changed, 8 insertions(+) -- 2.54.0
From: Peter Krempa <pkrempa@redhat.com> When probing of capabilities fails 'qemuProcessQMPStop' kills the qemu process, but also resets the error. The virResetLastError() was originally intended to reset the error from 'virProcessKillPainfully', it also resets any probing error resulting in the following: $ virsh start VM error: Failed to start domain 'VM' error: An error occurred, but the cause is unknown Keep the error clearing as this might be called on success but preserve the original error around it. Fixes: db7b0b12b7b07b8a0313161e3cbedb38a74e5954 Signed-off-by: Peter Krempa <pkrempa@redhat.com> --- src/qemu/qemu_process.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c index cbe5cb33d3..4c94ff3c91 100644 --- a/src/qemu/qemu_process.c +++ b/src/qemu/qemu_process.c @@ -10239,6 +10239,8 @@ static qemuMonitorCallbacks callbacks = { static void qemuProcessQMPStop(qemuProcessQMP *proc) { + virErrorPtr err; + if (proc->mon) { virObjectUnlock(proc->mon); g_clear_pointer(&proc->mon, qemuMonitorClose); @@ -10255,10 +10257,12 @@ qemuProcessQMPStop(qemuProcessQMP *proc) virDomainObjEndAPI(&proc->vm); if (proc->pid != 0) { + virErrorPreserveLast(&err); VIR_DEBUG("Killing QMP caps process %lld", (long long)proc->pid); virProcessKillPainfully(proc->pid, true); virResetLastError(); proc->pid = 0; + virErrorRestore(&err); } if (proc->pidfile) -- 2.54.0
From: Peter Krempa <pkrempa@redhat.com> While we don't normally try to re-wrap errors to be more descriptive, probing of qemu capabilities is a special case as the errors can be cryptic and don't give users enough information about what's going on. An example is (obtained from a patched qemu which accidentally returned a broken QMP schema): $ virsh start VM error: Failed to start domain 'VM' error: internal error: Duplicate hash table key 'str' The error is unusable. On the other hand in 'virQEMUCapsLogProbeFailure' we do special logging of the caps probing failure with a custom UUID so that it can be easily found in the logs which reports a much better error message. Re-raise the same as a normal error too so that the user gets similar experience: $ virsh start VM error: Failed to start domain 'VM' error: internal error: Failed to probe capabilities for /home/user/git/qemu.git/build/qemu-system-x86_64: internal error: Duplicate hash table key 'str' This does mean though that the error is logged twice, once via the custom log entry and second via 'virReportError'. Given that this error is extremely unlikely, having two log entries IMO doesn't warant reworking the whole error reporting infra to allow passing extra metadata just for this one case. Signed-off-by: Peter Krempa <pkrempa@redhat.com> --- src/qemu/qemu_capabilities.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c index a28f87ad2b..be57dcad8e 100644 --- a/src/qemu/qemu_capabilities.c +++ b/src/qemu/qemu_capabilities.c @@ -6065,6 +6065,10 @@ virQEMUCapsLogProbeFailure(const char *binary) meta, _("Failed to probe capabilities for %1$s: %2$s"), binary, virGetLastErrorMessage()); + + virReportError(VIR_ERR_INTERNAL_ERROR, + _("Failed to probe capabilities for %1$s: %2$s"), + binary, virGetLastErrorMessage()); } -- 2.54.0
On a Thursday in 2026, Peter Krempa via Devel wrote:
See both patches for explanation and examples.
Peter Krempa (2): qemuProcessQMPStop: Preserve error when killing qemu used for capability probing virQEMUCapsLogProbeFailure: Reraise error with more information
src/qemu/qemu_capabilities.c | 4 ++++ src/qemu/qemu_process.c | 4 ++++ 2 files changed, 8 insertions(+)
Reviewed-by: Ján Tomko <jtomko@redhat.com> Jano
participants (2)
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Ján Tomko -
Peter Krempa