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

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Date:	Tue, 14 Jul 2015 17:15:36 -0400 (EDT)
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Laszlo Ersek <lersek@...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]

> 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?
I think SEC doesn't do any MMIO, so it should be enough to enable MTRRs
and set the default type to writeback.

In any case we're going to have to quirk it, because of the broken
guests in the wild.

Paolo
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