Re: [PATCH 1/1] qapi/dump: add allowed-by-guest feature to win-dmp

On Fri, Jul 31, 2026 at 05:50:01PM +0200, Denis V. Lunev wrote:
> Commit 1c0e259c5a35 ("dump: make win_dump_available() check vmcoreinfo
> for a Windows dump header") changed two things in a way that is visible
> to QMP clients but not to introspection:
> query-dump-guest-memory-capability now lists win-dmp only for a guest
> that has published a Windows dump header through the vmcoreinfo device,
> and dump-guest-memory, which shares win_dump_available(), rejects the
> format otherwise. Before that, both accepted win-dmp on any x86
> machine.

IIUC, the old behaviour of 'dump-guest-memory' would always return
an error if invoked with win-dmp on Linux guests, as the first
lines of create_win_dump used to be:

    if (s->guest_note_size != VMCOREINFO_WIN_DUMP_NOTE_SIZE32 &&
            s->guest_note_size != VMCOREINFO_WIN_DUMP_NOTE_SIZE64) {
        error_setg(errp, "win-dump: invalid vmcoreinfo note size");
        return;
    }

At most we've change the error message that is seen by the client
AFAICT.

IOW from a client facing view, what changed in 11.1.0 is simply that
'query-dump-guest-memory-capability' now accurately reflects what can
be used by 'dump-guest-memory'.


> A client that wants to select win-dmp automatically therefore cannot
> trust the capability query on its own: on an older QEMU it reports
> win-dmp for every x86 guest, Linux ones included, where the resulting
> dump is useless. libvirt ran into exactly this while picking a format
> for on_crash and watchdog triggered dumps, and has no way to tell the
> two behaviours apart.
> 
> Add an 'allowed-by-guest' feature to the win-dmp member of
> DumpGuestMemoryFormat so the fixed behaviour becomes discoverable.
> DumpGuestMemoryFormat is reachable from both
> query-dump-guest-memory-capability's return type and
> dump-guest-memory's arguments, so a single flag covers both halves of
> the change. Where the feature is absent, a reported win-dmp says
> nothing about the guest, and a client that needs the dump to be
> loadable afterwards should fall back to elf.
> 
> CC: Eric Blake <[email protected]>
> CC: Markus Armbruster <[email protected]>
> CC: "Marc-André Lureau" <[email protected]>
> Suggested-by: Daniel P. Berrangé <[email protected]>
> Signed-off-by: Denis V. Lunev <[email protected]>
> ---
> libvirt discussion:
> https://lists.libvirt.org/archives/list/[email protected]/thread/7QBRM2GOC7M5E7EAIE44JNWBSKNN62BC/
> 
>  qapi/dump.json | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/qapi/dump.json b/qapi/dump.json
> index 726b520870..690f3963fe 100644
> --- a/qapi/dump.json
> +++ b/qapi/dump.json
> @@ -38,6 +38,13 @@
>  # @win-dmp: Windows full crashdump format, can be used instead of ELF
>  #     converting (since 2.13)
>  #
> +# Features:
> +#
> +# @allowed-by-guest: If present, @win-dmp is listed by
> +#     `query-dump-guest-memory-capability`, and accepted by
> +#     `dump-guest-memory`, only when the guest has published a Windows
> +#     dump header through the vmcoreinfo device (since 11.1)
> +#
>  # Since: 2.0
>  ##
>  { 'enum': 'DumpGuestMemoryFormat',
> @@ -45,7 +52,7 @@
>        'elf',
>        'kdump-zlib', 'kdump-lzo', 'kdump-snappy',
>        'kdump-raw-zlib', 'kdump-raw-lzo', 'kdump-raw-snappy',
> -      'win-dmp' ] }
> +      { 'name': 'win-dmp', 'features': ['allowed-by-guest'] } ] }
>  
>  ##
>  # @dump-guest-memory:

Regardless of quibble over the commit message

  Reviewed-by: Daniel P. Berrangé <[email protected]>

With regards,
Daniel
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