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Date:   Sat, 21 Jul 2018 18:06:19 +0200
From:   Pavel Machek <pavel@ucw.cz>
To:     Joerg Roedel <joro@8bytes.org>
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>,
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        Denys Vlasenko <dvlasenk@redhat.com>,
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        Waiman Long <llong@redhat.com>,
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        jroedel@suse.de, Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
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        Namhyung Kim <namhyung@kernel.org>
Subject: Re: [PATCH 2/3] x86/entry/32: Check for VM86 mode in slow-path check
Message-ID: <20180721160619.GA19856@amd>
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Hi!

> The SWITCH_TO_KERNEL_STACK macro only checks for CPL =3D=3D 0 to
> go down the slow and paranoid entry path. The problem is
> that this check also returns true when coming from VM86
> mode. This is not a problem by itself, as the paranoid path
> handles VM86 stack-frames just fine, but it is not necessary
> as the normal code path handles VM86 mode as well (and
> faster).
>=20
> Extend the check to include VM86 mode. This also makes an
> optimization of the paranoid path possible.
>=20
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  arch/x86/entry/entry_32.S | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>=20
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index 010cdb4..2767c62 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -414,8 +414,16 @@
>  	andl	$(0x0000ffff), PT_CS(%esp)
> =20
>  	/* Special case - entry from kernel mode via entry stack */
> -	testl	$SEGMENT_RPL_MASK, PT_CS(%esp)
> -	jz	.Lentry_from_kernel_\@
> +#ifdef CONFIG_VM86
> +	movl	PT_EFLAGS(%esp), %ecx		# mix EFLAGS and CS
> +	movb	PT_CS(%esp), %cl
> +	andl	$(X86_EFLAGS_VM | SEGMENT_RPL_MASK), %ecx
> +#else
> +	movl	PT_CS(%esp), %ecx
> +	andl	$SEGMENT_RPL_MASK, %ecx
> +#endif
> +	cmpl	$USER_RPL, %ecx
> +	jb	.Lentry_from_kernel_\@

Would it make sense to jump to the slow path as we did, and them jump
back if VM86 is detected?

Because VM86 is not really used often these days, and moving partial
registers results in short code but IIRC it will be rather slow.

								Pavel
--=20
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo=
g.html

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