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

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Date:	Mon, 13 Jul 2015 09:32:06 +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 12/07/2015 20:59, Xiao Guangrong wrote:
> 
> 
> On 07/13/2015 01:33 AM, Alex Williamson wrote:
>> On Wed, 2015-05-13 at 14:42 +0800, Xiao Guangrong wrote:
>>> There are some bugs in current get_mtrr_type();
>>> 1: bit 1 of mtrr_state->enabled is corresponding bit 11 of
>>>     IA32_MTRR_DEF_TYPE MSR which completely control MTRR's enablement
>>>     that means other bits are ignored if it is cleared
>>>
>>> 2: the fixed MTRR ranges are controlled by bit 0 of
>>>     mtrr_state->enabled (bit 10 of IA32_MTRR_DEF_TYPE)
>>>
>>> 3: if MTRR is disabled, UC is applied to all of physical memory rather
>>>     than mtrr_state->def_type
>>>
>>> Signed-off-by: Xiao Guangrong <guangrong.xiao@...ux.intel.com>
>>> ---
>>>   arch/x86/kvm/mmu.c | 14 ++++++--------
>>>   1 file changed, 6 insertions(+), 8 deletions(-)
>>
>>
>> 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.

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:

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.

Paolo
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