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 16:53:18 -0500
From:   Benjamin Gilbert <bgilbert@...hat.com>
To:     Wei Huang <wei@...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 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.

--Benjamin Gilbert