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Subject: Re: [PATCH 05/39] x86/entry/32: Unshare NMI return path
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <1531308586-29340-6-git-send-email-joro@8bytes.org>
Date:   Thu, 12 Jul 2018 13:53:19 -0700
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Brian Gerst <brgerst@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Eduardo Valentin <eduval@amazon.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Will Deacon <will.deacon@arm.com>, aliguori@amazon.com,
        daniel.gruss@iaik.tugraz.at, hughd@google.com, keescook@google.com,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        "David H . Gutteridge" <dhgutteridge@sympatico.ca>, jroedel@suse.de
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To:     Joerg Roedel <joro@8bytes.org>
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> On Jul 11, 2018, at 4:29 AM, Joerg Roedel <joro@8bytes.org> wrote:
> 
> From: Joerg Roedel <jroedel@suse.de>
> 
> NMI will no longer use most of the shared return path,
> because NMI needs special handling when the CR3 switches for
> PTI are added.

Why?  What would go wrong?

How many return-to-usermode paths will we have?  64-bit has only one.

> This patch prepares for that.
> 
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> arch/x86/entry/entry_32.S | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index d35a69a..571209e 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -1017,7 +1017,7 @@ ENTRY(nmi)
> 
>    /* Not on SYSENTER stack. */
>    call    do_nmi
> -    jmp    .Lrestore_all_notrace
> +    jmp    .Lnmi_return
> 
> .Lnmi_from_sysenter_stack:
>    /*
> @@ -1028,7 +1028,11 @@ ENTRY(nmi)
>    movl    PER_CPU_VAR(cpu_current_top_of_stack), %esp
>    call    do_nmi
>    movl    %ebx, %esp
> -    jmp    .Lrestore_all_notrace
> +
> +.Lnmi_return:
> +    CHECK_AND_APPLY_ESPFIX
> +    RESTORE_REGS 4
> +    jmp    .Lirq_return
> 
> #ifdef CONFIG_X86_ESPFIX32
> .Lnmi_espfix_stack:
> -- 
> 2.7.4
>