Date: Tue, 8 Nov 2016 18:00:34 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Borislav Petkov <bp@suse.de>, Brian Gerst <brgerst@gmail.com>,
        Chen Yucong <slaoub@gmail.com>, Chris Metcalf <cmetcalf@mellanox.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>, Huang Rui <ray.huang@amd.com>,
        Jiri Slaby <jslaby@suse.cz>, Jonathan Corbet <corbet@lwn.net>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Shuah Khan <shuah@kernel.org>, Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH 2/4] x86: Prepare vm86 tasks to handle User-Mode
 Instruction Prevention
Message-ID: <20161108170034.GK3117@twins.programming.kicks-ass.net>
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On Tue, Nov 08, 2016 at 08:01:39AM -0800, Andy Lutomirski wrote:
> > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > index 0888a87..32b7301 100644
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -233,6 +233,16 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
> >                  */
> >                 memset(tss->io_bitmap, 0xff, prev->io_bitmap_max);
> >         }
> > +
> > +#if defined(CONFIG_VM86) && defined(CONFIG_X86_INTEL_UMIP)
> > +       if (next->vm86 && next->vm86->saved_sp0 && next->vm86->disable_x86_umip)
> > +               cr4_clear_bits(X86_CR4_UMIP);
> > +       else {
> > +               if (static_cpu_has(X86_FEATURE_UMIP))
> > +                       cr4_set_bits(X86_CR4_UMIP);
> > +       }
> > +#endif
> > +
> 
> NAK.  If this code is going to exist, it needs to be deeply buried in
> some unlikely if statement that already exists.  There's no good
> reason to penalize all context switches to support some nonsensical
> vm86 use case.

Agreed, now if instead vm86 get to emulate these instructions, this all
magically goes away..