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: <55A57F27.6090705@redhat.com>
Date:	Tue, 14 Jul 2015 23:29:11 +0200
From:	Laszlo Ersek <lersek@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	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>,
	"Jordan Justen (Intel address)" <jordan.l.justen@...el.com>
Subject: Re: MTRR setup in OVMF [was: PATCH v3 01/10 KVM: MMU: fix decoding
 cache type from MTRR]

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. 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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