On Mon, May 11, 2015 at 01:47:57PM +0100, Paul Durrant wrote:
Currently running make defconfig will leave the above turned off which, at least for the Xen/QEMU use-case, means VMs will boot very slowly. There seems to have been some concern over non-QEMU use of ATA_DMA about five years ago, but I can find no particular concern over ATA_PIO32. Given the increased use of QEMU and Xen in that period it would seem like a good idea to default both of these on now. This patch does that.
The DMA support is known to be not correct for real hardware - correct code would negotiate the DMA rate with the drive (and also do things like detecting a 66Mhz PATA cable). The code is only really useful on emulators and when one knows that the drive is really a SATA drive connected to a controller that is doing PATA emulation.
The 32bit PIO mode has a minimal speed improvement and it is known to not work on some real hardware controllers.
If one is concerned about VM boot speed, why not just use AHCI or virtio?
-Kevin