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: <20150714223704.12156.30765@jljusten-ivb>
Date:	Tue, 14 Jul 2015 15:37:04 -0700
From:	Jordan Justen <jordan.l.justen@...el.com>
To:	Laszlo Ersek <lersek@...hat.com>,
	"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>
Subject: Re: MTRR setup in OVMF [was: PATCH v3 01/10 KVM: MMU: fix decoding cache
 type from MTRR]

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?

-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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