Re: [PATCH] igvm: populate errp in stub functions

[Luigi Leonardi <leonardi@redhat.com>]

Hi Daniel,

On Tue, Jun 09, 2026 at 02:25:01PM +0100, Daniel P. Berrangé wrote:
>On Tue, Jun 09, 2026 at 03:20:14PM +0200, Luigi Leonardi wrote:
>> Use error_setg() to report that IGVM is not available, matching
>> the pattern used by other stubs in the tree.
>>
>> Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
>> Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
>> ---
>>  stubs/igvm.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/stubs/igvm.c b/stubs/igvm.c
>> index 9e9f683fc9..dfb85eb548 100644
>> --- a/stubs/igvm.c
>> +++ b/stubs/igvm.c
>> @@ -17,15 +17,18 @@ int qigvm_x86_get_mem_map_entry(int index,
>>                                  ConfidentialGuestMemoryMapEntry *entry,
>>                                  Error **errp)
>>  {
>> +    error_setg(errp, "IGVM not supported on this platform");
>>      return -1;
>>  }
>>
>>  int qigvm_x86_set_vp_context(void *data, int index, Error **errp)
>>  {
>> +    error_setg(errp, "IGVM not supported on this platform");
>>      return -1;
>>  }
>>
>>  int qigvm_directive_madt(QIgvm *ctx, const uint8_t *header_data, Error **errp)
>>  {
>> +    error_setg(errp, "IGVM not supported on this platform");
>>      return -1;
>>  }
>
>This is not wrong per-se, so on that  basis
>
>  Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
>
>but are any of these stubs actually reachable when IGVM is not
>enabled in the build ? Usually with stubs we find that one
>or two methods are the primary entrypoints which must return
>an error, at which point everything else becomes unreachable.
>The latter cases can just be g_assert_not_reached() as a sanity
>check that some unexpected codepath isn't calling in without
>checking status earlier. Those would thus would not need
>error_setg, nor a 'return' statement.

No, when IGVM is not enabled in the build it's not possible to trigger
any of these stubs. On top of that, I don't think it is possible to trigger
these stubs even when igvm is enabled. This is because the only architecture
that implements igvm support in qemu is x86.

To give more context: the igvm code is split between `backends/igvm.c` and
`target/i386/igvm.c`. The former contains arch-independent functions,
while the latter, x86 related functions.

Therefore, stubs will only become relevant when new architectures will be
supported.

I'm not sure about using asserts here, because with this patch [1] we'll
handle optional igvm directives. So, when we support aarch64 in igvm,
we could have some directives (eg: `IGVM_VHT_MADT`) that are
implemented for one architecture but not for a different one, resulting
in the stub being hit. So, if that directive is marked as optional in the
igvm file, we should not crash qemu.

[1] https://lore.kernel.org/all/20260609-igvm_optional-v2-2-b1f1f08dc40e@redhat.com/

HTH,
Luigi