linux-kernel - Re: [tip:x86/urgent] x86/mm/64: Fix reboot interaction with CR4.PCIDE

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Message-ID: <20171009084257.0a8ac06c@gandalf.local.home>
Date:   Mon, 9 Oct 2017 08:42:57 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     tip-bot for Andy Lutomirski <tipbot@...or.com>
Cc:     luto@...nel.org, rostedt@...dmis.org, hpa@...or.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org, tglx@...utronix.de,
        bp@...en8.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/mm/64: Fix reboot interaction with
 CR4.PCIDE

On Mon, 9 Oct 2017 04:36:01 -0700
tip-bot for Andy Lutomirski <tipbot@...or.com> wrote:

> Commit-ID:  924c6b900cfdf376b07bccfd80e62b21914f8a5a
> Gitweb:     https://git.kernel.org/tip/924c6b900cfdf376b07bccfd80e62b21914f8a5a
> Author:     Andy Lutomirski <luto@...nel.org>
> AuthorDate: Sun, 8 Oct 2017 21:53:05 -0700
> Committer:  Thomas Gleixner <tglx@...utronix.de>
> CommitDate: Mon, 9 Oct 2017 13:31:04 +0200
> 
> x86/mm/64: Fix reboot interaction with CR4.PCIDE
> 
> Trying to reboot via real mode fails with PCID on: long mode cannot
> be exited while CR4.PCIDE is set.  (No, I have no idea why, but the
> SDM and actual CPUs are in agreement here.)  The result is a GPF and
> a hang instead of a reboot.
> 
> I didn't catch this in testing because neither my computer nor my VM
> reboots this way.  I can trigger it with reboot=bios, though.
> 
> Fixes: 660da7c9228f ("x86/mm: Enable CR4.PCIDE on supported systems")

> Reported-and-tested-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Didn't someone somewhere say not to combine tags? Have them always be
separate. I noticed they were separate when Andy submitted it, but I'm
assuming that Thomas combined it.

-- Steve


> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Borislav Petkov <bp@...en8.de>
> Link: https://lkml.kernel.org/r/f1e7d965998018450a7a70c2823873686a8b21c0.1507524746.git.luto@kernel.org
> 
> ---
>  arch/x86/kernel/reboot.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> index 54180fa..add33f6 100644
> --- a/arch/x86/kernel/reboot.c
> +++ b/arch/x86/kernel/reboot.c
> @@ -105,6 +105,10 @@ void __noreturn machine_real_restart(unsigned int type)
>  	load_cr3(initial_page_table);
>  #else
>  	write_cr3(real_mode_header->trampoline_pgd);
> +
> +	/* Exiting long mode will fail if CR4.PCIDE is set. */
> +	if (static_cpu_has(X86_FEATURE_PCID))
> +		cr4_clear_bits(X86_CR4_PCIDE);
>  #endif
>  
>  	/* Jump to the identity-mapped low memory code */