hw/ide/piix: Add some documentation to pci_piix3_xen_ide_unplug()

The comment is based on commit message
ae4d2eb273b167dad748ea4249720319240b1ac2 'xen-platform: add missing disk
unplug option'. Since it seems to describe design decisions and
limitations that still apply it seems worth having.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20220513180957.90514-3-shentey@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

diff --git a/hw/ide/piix.c b/hw/ide/piix.c
index 2345fe9e1dd0035d31995130be63efd81ccf6e48..bc1b37512ab32ba46d19ceb06f2529274850ae63 100644 (file)
--- a/hw/ide/piix.c
+++ b/hw/ide/piix.c
@@ -173,6 +173,17 @@ static void pci_piix_ide_realize(PCIDevice *dev, Error **errp)
     }
 }
 
+/*
+ * The Xen HVM unplug protocol [1] specifies a mechanism to allow guests to
+ * request unplug of 'aux' disks (which is stated to mean all IDE disks,
+ * except the primary master).
+ *
+ * NOTE: The semantics of what happens if unplug of all disks and 'aux' disks
+ *       is simultaneously requested is not clear. The implementation assumes
+ *       that an 'all' request overrides an 'aux' request.
+ *
+ * [1] https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/hvm-emulated-unplug.pandoc
+ */
 int pci_piix3_xen_ide_unplug(DeviceState *dev, bool aux)
 {
     PCIIDEState *pci_ide;