ATA does not only have the WCACHE enabled bit in identify word 85, but also a WCACHE supported bit in word 82. While the Linux kernel is fine with the latter at least hdparm also needs the former before correctly displaying the cache settings. There's also a non-zero chance other operating systems are more picky in their volatile write cache detection.
Signed-off-by: Christoph Hellwig <h...@lst.de> Index: qemu/hw/ide/core.c =================================================================== --- qemu.orig/hw/ide/core.c 2010-10-04 15:22:26.848003542 +0200 +++ qemu/hw/ide/core.c 2010-10-04 15:24:11.900255468 +0200 @@ -146,8 +146,8 @@ static void ide_identify(IDEState *s) put_le16(p + 68, 120); put_le16(p + 80, 0xf0); /* ata3 -> ata6 supported */ put_le16(p + 81, 0x16); /* conforms to ata5 */ - /* 14=NOP supported, 0=SMART supported */ - put_le16(p + 82, (1 << 14) | 1); + /* 14=NOP supported, 5=WCACHE supported, 0=SMART supported */ + put_le16(p + 82, (1 << 14) | (1 << 5) | 1); /* 13=flush_cache_ext,12=flush_cache,10=lba48 */ put_le16(p + 83, (1 << 14) | (1 << 13) | (1 <<12) | (1 << 10)); /* 14=set to 1, 1=SMART self test, 0=SMART error logging */