Re: [Qemu-devel] Wierd hack to sound/pci/intel8x0.c

[Denis V. Lunev]

On 11/6/11 6:51 PM, Avi Kivity wrote:
> The recently merged 228cf79376f1 ("ALSA: intel8x0: Improve performance
> in virtual environment") is hacky and somewhat wrong.
>
> First, the detection code
>
> +       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
> +       }
> +
>
> is incorrect.  It detects that you're running in a guest, but that
> doesn't imply that the device you're accessing is emulated.  It may be a
> host device assigned to the guest; presumably the optimization you apply
> doesn't work for real devices.
>
> Second, the optimization itself looks fishy:
>
>          spin_lock(&chip->reg_lock);
>          do {
>                  civ = igetbyte(chip, ichdev->reg_offset + ICH_REG_OFF_CIV);
>                  ptr1 = igetword(chip, ichdev->reg_offset +
> ichdev->roff_picb);
>                  position = ichdev->position;
>                  if (ptr1 == 0) {
>                          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--);
>
>
> Why is the emulated device timing out?  Can't the emulation be fixed to
> behave like real hardware?
>
> Last, please copy address@hidden on such issues.
The problem is that emulation can not be fixed.

How this is working for real hardware? You get data from real sound card 
register.
The scheduling is off at the moment thus you can not be re-scheduled.

In the virtual environment the situation is different. Any IO emulation 
is expensive.
The processor is switched from guest to hypervisor and further to 
emulation process
takes a lot of time.  This time is enough to obtain different value on 
next register read.
That's why this code is really timed out. Please also note that host 
scheduler also
plays his games and could schedule out VCPU thread.

The problem could be potentially fixed reducing precision of PICB emulation,
but this results in lower sound quality.

This kludge has been written this way in order not to break legacy card 
for which we
do not have an access. The code reading PICB/CIV registers second time 
was added
to fix issues on unknown for now platform and it looks not possible how 
to find/test
against this platform now. We have checked Windows drivers written by 
Intel/AMD
(32/64 bit) and MacOS ones. There is no second reading of CIV/PICB 
inside. We
hope that this is relay needed only for some rare hadware devices.

The only thing we can is to improve detection code. Suggestions are welcome.

Regards,
    Den