linux-kernel - Re: [PATCH] X86: don't print a warning when MTRR are blank and running in KVM

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Date:	Fri, 22 Feb 2008 13:48:30 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Joerg Roedel <joerg.roedel@....com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, tglx@...utronix.de,
	mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] X86: don't print a warning when MTRR are blank and
	running in KVM

* Joerg Roedel <joerg.roedel@....com> wrote:

> > ok. Then i guess we should just leave the warning and the backtrace 
> > in place until they get a fix done?
> 
> No. I don't agree. The MTRRs are set up by the BIOS because it knows 
> the hardware best (I know this is only true in theory). The OS should 
> basically trust the BIOS MTRR settings. If the OS can't trust the 
> BIOS, like on real hardware, the check and the warning is necessary. 
> If the OS can trust the BIOS, like running in a KVM or QEMU virtual 
> machine, it should just ignore the case where all MTRRs are blank.

well, i can agree with the warning being pointless on KVM paravirt, but 
it's not pointless in the Qemu context.

we fundamentall _dont_ trust the BIOS's MTRR settings, just check the 
very function that you modified: mtrr_trim_uncached_memory() - that 
function handles _exactly_ a case where a BIOS messes up MTRR's. This 
problem has caused real user-side breakage and just not emitting a 
warning because Qemu is lazy to emulate a proper PC is not a strong 
enough argument.

so i've applied the KVM bits but the warning stays for Qemu.

	Ingo
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