linux-kernel - Re: [PATCH v7 00/27] x86, boot, 64bit: Add support for loading ramdisk and bzImage above 4G

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Message-ID: <20121225115226.GD7720@liondog.tnic>
Date:	Tue, 25 Dec 2012 12:52:26 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 00/27] x86, boot, 64bit: Add support for loading
 ramdisk and bzImage above 4G

On Mon, Dec 24, 2012 at 05:06:01PM -0800, H. Peter Anvin wrote:
> Could this be the ljmpq problem that Borislav reported
> (Intel implemented ljmpq, AMD didn't, and I was tempted by a
> micro-optimization which broke AMD which made it into the patchset)?

It has to be. Just booted Yinghai's -v8 in kvm on an AMD host and it
worked fine.

With the change below it keeps rebooting like I reported earlier. I'd go
out on a limb here and guess that the guest is triple-faulting due to an
unhandled #GP caused by an invalid opcode or similar.

---
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index d94f6d68be2a..1842d30c96a2 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -279,11 +279,8 @@ ENTRY(secondary_startup_64)
         *      REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
         *              address given in m16:64.
         */
-       movq    initial_code(%rip),%rax
        pushq   $0              # fake return address to stop unwinder
-       pushq   $__KERNEL_CS    # set correct cs
-       pushq   %rax            # target address in negative space
-       lretq
+       rex64 ljmp *initial_code(%rip)
 
 #ifdef CONFIG_HOTPLUG_CPU
 /*

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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