linux-kernel - Re: EFI mixed mode + perf = rampant triple faults

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Message-ID: <20150115194127.GB12079@codeblueprint.co.uk>
Date:	Thu, 15 Jan 2015 19:41:27 +0000
From:	Matt Fleming <matt@...sole-pimps.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: EFI mixed mode + perf = rampant triple faults

On Wed, 14 Jan, at 10:27:47AM, Andy Lutomirski wrote:
> 
> How are you manually triggering an MCE?  I've been playing with some
> MCE stuff recently, but the only reasonably reliable way I know of to
> trigger an MCE is using WHEA, and I don't have a box with WHEA, and I
> assume your ASUS T100 doesn't either.
 
As Borislav mentions, I used 'int $18', solely to trigger the 64-bit
exception handler code paths in the middle of the EFI mixed mode code.

> > Where this won't work so well is at boot time before we jump to the
> > kernel proper. There, we still need to restore the firmware's GDT so
> > that interrupts are serviced correctly before ExitBootServices() (in
> > particular, ia32 Tianocore assumes __KERNEL_CS is a 32-bit CS).
> 
> Tianocore makes assumptions about the kernel's GDT layout?  Yuck.

No, but 32-bit Tianocore does rely on the second GDT entry being a
32-bit CS.

It has no knowledge of Linux's GDT layout.

-- 
Matt Fleming, Intel Open Source Technology Center
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