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

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Message-ID: <55A62E8A.9060708@redhat.com>
Date:	Wed, 15 Jul 2015 11:57:30 +0200
From:	Laszlo Ersek <lersek@...hat.com>
To:	Jordan Justen <jordan.l.justen@...el.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Alex Williamson <alex.williamson@...hat.com>, gleb@...nel.org,
	mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	edk2-devel list <edk2-devel@...ts.sourceforge.net>
Subject: Re: MTRR setup in OVMF [was: PATCH v3 01/10 KVM: MMU: fix decoding
 cache type from MTRR]

On 07/15/15 00:37, Jordan Justen wrote:
> On 2015-07-14 14:29:11, Laszlo Ersek wrote:
>> On 07/14/15 23:15, Paolo Bonzini wrote:
>>>> The long delay that Alex reported (for the case when all guest memory
>>>> was set to UC up-front) is due to the fact that the SEC phase of OVMF
>>>> decompresses an approximately 1712 KB sized, LZMA-compressed blob, to
>>>> approx. 896 KB worth of PEI drivers and 8192 KB worth of DXE and UEFI
>>>> drivers -- and this decompression is extremely memory-intensive.
>>>>
>>>> (When Jordan implemented that reset vector first, we saw similar
>>>> performance degradation on AMD hosts (albeit not due to MTRR but due to
>>>> page attributes). See
>>>> <https://github.com/tianocore/edk2/commit/98f378a7>. I'm only mentioning
>>>> it here because it makes me appreciate the current problem report.)
>>>>
>>>> Anyway, the reset vector's page table building is implemented in
>>>> "OvmfPkg/ResetVector/Ia32/PageTables64.asm". The decompression in SEC
>>>> can be found in "OvmfPkg/Sec/SecMain.c", function DecompressMemFvs().
>>>
>>> Perhaps the OVMF reset vector should initialize the MTRRs for the BSP?
>>
>> That's an idea, yes, if someone feels sufficiently drawn to writing
>> assembly.
> 
> Maybe we can use MtrrLib in the SEC C code?

If the page table building in the reset vector is not too costly with
UC, then yes, it should suffice if MtrrLib was put to use first just
before the decompression in SEC.

Thanks
Laszlo

> -Jordan
> 
>> Complications:
>> - the reset vector is specific to OvmfPkg only in the OvmfPkgX64.dsc
>>   build
>> - it needs to be determined what memory to cover.
>>
>>> I think SEC doesn't do any MMIO, so it should be enough to enable MTRRs
>>> and set the default type to writeback.
>>
>> Seems safe to me, off the top of my head (and testing could confirm /
>> disprove quickly).
>>
>>> In any case we're going to have to quirk it, because of the broken
>>> guests in the wild.
>>
>> Thanks.
>> Laszlo

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