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

Xiao Guangrong
2015-07-13T14:50:09Z

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?