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Date: Mon, 09 Mar 2015 10:42:40 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>,
        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>, Oleg Nesterov <oleg@redhat.com>,
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        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)
References: <1425909943-14687-1-git-send-email-dvlasenk@redhat.com> <CA+55aFzck8nuV06AZhhFXgUQEsujZCtufw0jY4iswJUggh6ruA@mail.gmail.com>
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On 03/09/2015 09:08 AM, Linus Torvalds wrote:
> 
> 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.
> 

Maybe:

	testb $(X86_EFLAGS_VM-16), PT_EFLAGS+2(%esp)

> 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.

The question is if avoiding looking at a field on the stack matters at all.

	-hpa