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Virtio spec 1.1 (and earlier), 5.2.5.2 Driver Requirements: Device Initialization: "Devices SHOULD always offer VIRTIO_BLK_F_FLUSH, and MUST offer it if they offer VIRTIO_BLK_F_CONFIG_WCE" Currently F_CONFIG_WCE and F_WCE are not connected to each other. Qemu will advertise F_CONFIG_WCE if config-wce argument is set for virtio-blk device. And F_WCE is advertised only if underlying block backend actually has it's caching enabled. Fix this by advertising F_WCE if F_CONFIG_WCE is also advertised. To preserve backwards compatibility with newer machine types make this behaviour governed by "x-enable-wce-if-config-wce" virtio-blk-device property and introduce hw_compat_4_2 with new property being off by default for all machine types <= 4.2 (but don't introduce 4.3 machine type itself yet). Signed-off-by: Evgeny Yakovlev <wrfsh@yandex-team.ru> Message-Id: <1572978137-189218-1-git-send-email-wrfsh@yandex-team.ru> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>