ALSA: intel8x0: Improve performance in virtual environment

v3: detection code is x86 and KVM specific, hide it under ifdef
v2: add detection for virtual environments (KVM and Parallels)

This patch is intended to improve performance in virtualized environments
like Parallels Desktop or KVM/VirtualBox/QEMU (virtual ICH/AC97 audio).

I/O access is very time-expensive operation in virtual world: VCPU
can be rescheduled and in the worst case we get more than 10ms delay on
each I/O access.

In the virtual environment loop exit rule
(old_civ == current_civ && old_picb == current_picb) is never satisfied,
because old_picb is never the same as current_picb due to delay inspired
by reading current_civ. As a result loop ended by timeout and we get 10x
more I/O operations.

Experimental data from Prallels Desktop 7, RHEL6 guest (I/O ops per
second):

Original code:
In Port    Counter         Callback
   f014      41550         fffff00000179d00 ac97_bm_read_civ+0x000
   f018      41387         fffff0000017a580 ac97_bm_read_picb+0x000

With patch:
In Port    Counter         Callback
   f014       4090         fffff00000179d00 ac97_bm_read_civ+0x000
   f018       1964         fffff0000017a580 ac97_bm_read_picb+0x000

Signed-off-by: Konstantin Ozerkov <kozerkov@parallels.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/intel8x0.c

@@ -42,6 +42,12 @@
 #include <asm/pgtable.h>
 #include <asm/cacheflush.h>
 
+#ifdef CONFIG_KVM_GUEST
+#include <asm/kvm_para.h>
+#else
+#define kvm_para_available() (0)
+#endif
+
 MODULE_AUTHOR("Jaroslav Kysela <perex@perex.cz>");
 MODULE_DESCRIPTION("Intel 82801AA,82901AB,i810,i820,i830,i840,i845,MX440; SiS 7012; Ali 5455");
 MODULE_LICENSE("GPL");
@@ -77,6 +83,7 @@ static int buggy_semaphore;
 static int buggy_irq = -1; /* auto-check */
 static int xbox;
 static int spdif_aclink = -1;
+static int inside_vm = -1;
 
 module_param(index, int, 0444);
 MODULE_PARM_DESC(index, "Index value for Intel i8x0 soundcard.");
@@ -94,6 +101,8 @@ module_param(xbox, bool, 0444);
 MODULE_PARM_DESC(xbox, "Set to 1 for Xbox, if you have problems with the AC'97 codec detection.");
 module_param(spdif_aclink, int, 0444);
 MODULE_PARM_DESC(spdif_aclink, "S/PDIF over AC-link.");
+module_param(inside_vm, bool, 0444);
+MODULE_PARM_DESC(inside_vm, "KVM/Parallels optimization.");
 
 /* just for backward compatibility */
 static int enable;
@@ -400,6 +409,7 @@ struct intel8x0 {
 	unsigned buggy_irq: 1;		/* workaround for buggy mobos */
 	unsigned xbox: 1;		/* workaround for Xbox AC'97 detection */
 	unsigned buggy_semaphore: 1;	/* workaround for buggy codec semaphore */
+	unsigned inside_vm: 1;		/* enable VM optimization */
 
 	int spdif_idx;	/* SPDIF BAR index; *_SPBAR or -1 if use PCMOUT */
 	unsigned int sdm_saved;	/* SDM reg value */
@@ -1065,8 +1075,11 @@ static snd_pcm_uframes_t snd_intel8x0_pcm_pointer(struct snd_pcm_substream *subs
 			udelay(10);
 			continue;
 		}
-		if (civ == igetbyte(chip, ichdev->reg_offset + ICH_REG_OFF_CIV) &&
-		    ptr1 == igetword(chip, ichdev->reg_offset + ichdev->roff_picb))
+		if (civ != igetbyte(chip, ichdev->reg_offset + ICH_REG_OFF_CIV))
+			continue;
+		if (chip->inside_vm)
+			break;
+		if (ptr1 == igetword(chip, ichdev->reg_offset + ichdev->roff_picb))
 			break;
 	} while (timeout--);
 	ptr = ichdev->last_pos;
@@ -2984,6 +2997,10 @@ static int __devinit snd_intel8x0_create(struct snd_card *card,
 	if (xbox)
 		chip->xbox = 1;
 
+	chip->inside_vm = inside_vm;
+	if (inside_vm)
+		printk(KERN_INFO "intel8x0: enable KVM optimization\n");
+
 	if (pci->vendor == PCI_VENDOR_ID_INTEL &&
 	    pci->device == PCI_DEVICE_ID_INTEL_440MX)
 		chip->fix_nocache = 1; /* enable workaround */
@@ -3226,6 +3243,14 @@ static int __devinit snd_intel8x0_probe(struct pci_dev *pci,
 			buggy_irq = 0;
 	}
 
+	if (inside_vm < 0) {
+		/* detect KVM and Parallels virtual environments */
+		inside_vm = kvm_para_available();
+#if defined(__i386__) || defined(__x86_64__)
+		inside_vm = inside_vm || boot_cpu_has(X86_FEATURE_HYPERVISOR);
+#endif
+	}
+
 	if ((err = snd_intel8x0_create(card, pci, pci_id->driver_data,
 				       &chip)) < 0) {
 		snd_card_free(card);