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Date:   Tue, 7 Apr 2020 22:18:40 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Nadav Amit <nadav.amit@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, hch@infradead.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        mingo <mingo@redhat.com>, bp <bp@alien8.de>, hpa@zytor.com,
        x86 <x86@kernel.org>, "Kenneth R. Crudup" <kenny@panix.com>,
        Jessica Yu <jeyu@kernel.org>,
        Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Tony Luck <tony.luck@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        jannh@google.com, keescook@chromium.org, David.Laight@aculab.com,
        Doug Covelli <dcovelli@vmware.com>, mhiramat@kernel.org
Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation
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On Tue, Apr 07, 2020 at 11:27:54PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 07, 2020 at 02:22:11PM -0700, Nadav Amit wrote:
> > Anyhow, I do not think it is the only use-case which is not covered by your
> > patches (even considering CRs/DRs alone). For example, there is no kernel
> > function to turn on CR4.VMXE, which is required to run hypervisors on x86.
> 
> That needs an exported function; there is no way we'll allow random
> writes to CR4, there's too much dodgy stuff in there.

And this clearly shows while trying to cater to anyone doing hardware
virt out of tree is just a disaster and we need to stick to our
traditional line that out of tree modules don't matter, if you care
about your module bring it upstream.  Especially as we have a perfectly
fine upstream module for just about every variant of hardware
virtualization that can be extended for the needs of other hypervisors.