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

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Date:	Mon, 13 Jul 2015 17:13:00 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.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 13/07/2015 16:45, Xiao Guangrong wrote:
>> +    /* 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).
> 
> :(
> 
> Based on the SDM, UC is applied to all memory rather than default-type
> if MTRR is disabled.

There are two issues I think.  One is that I cannot find in the current
code that "UC is applied to all memory rather than default-type if MTRR
is disabled".  mtrr_default_type unconditionally looks at
mtrr_state->deftype.

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

Yes.

>>
>> 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?

That's what quirks are for...  The firmware should still be fixed of course.

Paolo
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