linux-kernel - Re: [PATCH 1/1] x86/boot/compressed/64: Set EFER.LME=1 in 32-bit trampoline code before returning to long mode

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Date:   Mon, 7 Jan 2019 14:03:15 -0600
From:   Wei Huang <wei@...hat.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        hpa@...or.com, linux-kernel@...r.kernel.org,
        linux-x86_64@...r.kernel.org, bgilbert@...hat.com
Subject: Re: [PATCH 1/1] x86/boot/compressed/64: Set EFER.LME=1 in 32-bit
 trampoline code before returning to long mode

[adding lkml and linux-x86_64]

On 1/7/19 2:25 AM, Kirill A. Shutemov wrote:
> On Fri, Jan 04, 2019 at 05:44:11AM +0000, Wei Huang wrote:
>> In some old AMD KVM implementation, guest's EFER.LME bit is cleared by KVM
>> when the hypervsior detects guest sets CR0.PG to 0. This causes guest OS
>> to reboot when it tries to return from 32-bit trampoline code because CPU
>> is in incorrect state: CR4.PAE=1, CR0.PG=1, CS.L=1, but EFER.LME=0.
>> As a precaution, this patch sets EFER.LME=1 as part of long mode
>> activation procedure. This extra step won't cause any harm when Linux is
>> booting on bare-metal machine.
>>
>> Signed-off-by: Wei Huang <wei@...hat.com>
> 
> Thanks for tracking this down.

BTW I think this patch _might_ be related the recent reboot issue
reported in https://lkml.org/lkml/2018/7/1/836 since the symptoms are
exactly the same.

> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Fixes: 34bbb0009f3b ("x86/boot/compressed: Enable 5-level paging during decompression stage")
>