Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Oleg Nesterov <oleg@redhat.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)
Date: Mon, 9 Mar 2015 09:08:11 -0700	[thread overview]
Message-ID: <CA+55aFzck8nuV06AZhhFXgUQEsujZCtufw0jY4iswJUggh6ruA@mail.gmail.com> (raw)
In-Reply-To: <1425909943-14687-1-git-send-email-dvlasenk@redhat.com>

On Mon, Mar 9, 2015 at 7:05 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>     New:
>      1e6:   0f ba 64 24 38 11       btl    $0x11,0x38(%esp)

btl? Really?

Why isn't that just

    testb $2,0x3a(%esp)

which is both smaller and quite a bit faster on older machines.

Sure, the btl is easier to explain in the source code, but instead of this:

> +       btl     $X86_EFLAGS_VM_BIT,PT_EFLAGS(%esp)

you'd have to add a comment, like

    testb $2, PT_EFLAGS+2(%esp)  # X86_EFLAGS_VM_BIT

or something.

Or just at least *partially* do what we used to do, and make it all be

    movb  PT_EFLAGS+2(%esp),%al
    andb $2,%al
    orb PT_CS(%esp),%al
    testb $3,%al
    je restore_nocheck
    testb $SEGMENT_TI_MASK,PT_OLDSS(%esp)
    jne ldt_ss

which still avoids looking at SS unless needed, and is smaller and
faster than the btl, afaik.

                       Linus