[PATCH 1/1] ata_piix: defer disks to the Hyper-V paravirtualised drivers by default

From: Andy Whitcroft
Date: Thu Mar 29 2012 - 13:28:58 EST

When we are hosted on a Hyper-V hypervisor the guest disks are exposed
both via the Hyper-V paravirtualised drivers and via an emulated SATA disk
controller. We want to use the paravirtualised drivers where possible as
they are much more performant. The Hyper-V paravirtualised drivers only
expose the virtual hard disk devices, the CDROM/DVD devices must still
be enumerated on the virtualised SATA controller. As we have no control
over kernel probe order for these two drivers especially when one driver
is builtin to the kernel and the other a module, we need to prevent the
ata_piix driver from claiming the disks devices by default when running
on a Hyper-V hypervisor.

When enumerating the drives look at the aquired device ID and if it
appears to be a disk device then report it as disconnected. Limit this
behaviour to when we have detected a Hyper-V hypervisor. Finally allow
this behaviour to be overriden via a new module parameter.

BugLink: http://bugs.launchpad.net/bugs/929545
BugLink: http://bugs.launchpad.net/bugs/942316
Signed-off-by: Andy Whitcroft <apw@xxxxxxxxxxxxx>
drivers/ata/ata_piix.c | 29 +++++++++++++++++++++++++++++
1 files changed, 29 insertions(+), 0 deletions(-)

This was discovered when doing some boot testing on a Hyper-V
host. Performance was found to be bad as out builtin ata_piix
driver was grabbing the disks. I am Cc:ing a couple of the
Microsoft people who worked on the HV driver set as well.

Note that the device id data is converted from device to host
order in the caller which makes examining the data problematic.
As this code only make sense on X86 and we know we are in
matching order we can access the data without first fixing it.
Alternativly we could add a new callback from the ata core
after the code has been fixed for validation.



diff --git a/drivers/ata/ata_piix.c b/drivers/ata/ata_piix.c
index 68013f9..64895f8 100644
--- a/drivers/ata/ata_piix.c
+++ b/drivers/ata/ata_piix.c
@@ -94,6 +94,9 @@
#include <scsi/scsi_host.h>
#include <linux/libata.h>
#include <linux/dmi.h>
+#ifdef CONFIG_X86
+#include <asm/hypervisor.h>

#define DRV_NAME "ata_piix"
#define DRV_VERSION "2.13"
@@ -188,6 +191,29 @@ static int piix_pci_device_resume(struct pci_dev *pdev);

static unsigned int in_module_init = 1;

+static int prefer_ms_hyperv = 1;
+unsigned int ata_piix_read_id(struct ata_device *dev,
+ struct ata_taskfile *tf, u16 *id)
+ int ret = ata_do_dev_read_id(dev, tf, id);
+#ifdef CONFIG_X86
+ /* XXX: note that the device id is in little-endian order, the caller
+ * will shift it to host order, but we are working with little-endian.
+ * As this is _only_ used on x86 we can actually directly access it
+ * as host is also little-endian.
+ */
+ if (!ret && prefer_ms_hyperv && x86_hyper == &x86_hyper_ms_hyperv &&
+ ata_id_is_ata(id)) {
+ ata_dev_printk(dev, KERN_WARNING, "ATA disk ignored deferring to Hyper-V paravirt driver\n");
+ }
+ return ret;
static const struct pci_device_id piix_pci_tbl[] = {
/* Intel PIIX3 for the 430HX etc */
{ 0x8086, 0x7010, PCI_ANY_ID, PCI_ANY_ID, 0, 0, piix_pata_mwdma },
@@ -359,6 +385,7 @@ static struct ata_port_operations piix_pata_ops = {
.set_piomode = piix_set_piomode,
.set_dmamode = piix_set_dmamode,
.prereset = piix_pata_prereset,
+ .read_id = ata_piix_read_id,

static struct ata_port_operations piix_vmw_ops = {
@@ -1703,3 +1730,5 @@ static void __exit piix_exit(void)

+module_param(prefer_ms_hyperv, int, 0);

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