linux-kernel - Re: [PATCH v3 01/10] KVM: MMU: fix decoding cache type from MTRR

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Date:	Mon, 13 Jul 2015 22:45:06 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Alex Williamson <alex.williamson@...hat.com>
CC:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 01/10] KVM: MMU: fix decoding cache type from MTRR


On 07/13/2015 03:32 PM, Paolo Bonzini wrote:

>>> I'm seeing a significant regression in boot performance on Intel
>>> hardware with assigned devices that bisects back to this patch.  There's
>>> a long delay with Seabios between the version splash and execution of
>>> option ROMs, and a _very_ long delay with OVMF before the display is
>>> initialized.  The delay is long enough that users are reporting their
>>> previously working VM is hung with 100% CPU usage on v4.2-rc1.  Thanks,
>>>
>>
>> Alex, thanks for your report. I will try to reproduce and fix it asap.
>

I have reproduced the issue and I think Bandan and Paolo is correct.

> The code that Bandan pointed out
>
> +	/* MTRR is completely disabled, use UC for all of physical memory. */
> +	if (!(mtrr_state->enabled & 0x2))
> +		return MTRR_TYPE_UNCACHABLE;
>
> actually disappears in commit fa61213746a7 (KVM: MTRR: simplify
> kvm_mtrr_get_guest_memory_type, 2015-06-15).  Should mtrr_default_type
> actually be something like this:

:(

Based on the SDM, UC is applied to all memory rather than default-type
if MTRR is disabled.

If i changed the code to:
   	if (!(mtrr_state->enabled & 0x2))
  		return mtrr_state->def_type;
the result is the same as before.

However, fast boot came back if "return 0xFF" here. So fast boot expects
that the memory type is WB.

>
> static u8 mtrr_default_type(struct kvm_mtrr *mtrr_state)
> {
>          if (mtrr_is_enabled(mtrr_state))
>                  return mtrr_state->deftype & IA32_MTRR_DEF_TYPE_TYPE_MASK;
>          else
>                  return MTRR_TYPE_UNCACHABLE;
> }
>
> ?  Then it's easy to add a quirk that makes the default WRITEBACK until
> MTRRs are enabled.

It is the wrong configure in OVMF... shall we need to adjust KVM to satisfy
OVMF?

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