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    virtio-blk: do not relay a previous driver's WCE configuration to the current · ef5bc962
    Paolo Bonzini 提交于
    The following sequence happens:
    - the SeaBIOS virtio-blk driver does not support the WCE feature, which
    causes QEMU to disable writeback caching
    
    - the Linux virtio-blk driver resets the device, finds WCE is available
    but writeback caching is disabled; tells block layer to not send cache
    flush commands
    
    - the Linux virtio-blk driver sets the DRIVER_OK bit, which causes
    writeback caching to be re-enabled, but the Linux virtio-blk driver does
    not know of this side effect and cache flushes remain disabled
    
    The bug is at the third step.  If the guest does know about CONFIG_WCE,
    QEMU should ignore the WCE feature's state.  The guest will control the
    cache mode solely using configuration space.  This change makes Linux
    do flushes correctly, but Linux will keep SeaBIOS's writethrough mode.
    
    Hence, whenever the guest is reset, the cache mode of the disk should
    be reset to whatever was specified in the "-drive" option.  With this
    change, the Linux virtio-blk driver finds that writeback caching is
    enabled, and tells the block layer to send cache flush commands
    appropriately.
    
    Reported-by: Rusty Russell <rusty@au1.ibm.com
    Cc: qemu-stable@nongnu.org
    Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
    ef5bc962
virtio-blk.c 22.1 KB