Re: [Xen-devel] [SeaBIOS] [PATCH] Make CONFIG_ATA_DMA and CONFIG_ATA_PIO32 default on

[Kevin O'Connor <kevin@xxxxxxxxxxxx>]

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

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