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 16:02:23 -0600
From:   Wei Huang <wei@...hat.com>
To:     Benjamin Gilbert <bgilbert@...hat.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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
Subject: Re: [PATCH 1/1] x86/boot/compressed/64: Set EFER.LME=1 in 32-bit
 trampoline code before returning to long mode



On 1/7/19 3:53 PM, Benjamin Gilbert wrote:
> On Mon, Jan 07, 2019 at 02:03:15PM -0600, Wei Huang wrote:
>> 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.
> 
> The problem in that case turned out to be https://lkml.org/lkml/2018/7/4/723
> which was fixed by d503ac531a.

OK, then it is a different problem. For this specific patch, without it,
the latest kernel can't boot on RHEL6 (and other old KVM distros) as a
guest VM on AMD box.

> 
> --Benjamin Gilbert
>