Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation

Peter Zijlstra
2020-04-08T08:46:38Z

On Wed, Apr 08, 2020 at 12:12:14AM +0200, Paolo Bonzini wrote:
> On 07/04/20 23:27, 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.
>
> native_write_cr4 and pv_ops (through which you can do write_cr4) are
> both exported, and so is cpu_tlbstate which is used by __cr4_set_bits
> and friends. Am I missing something glaringly obvious?

cpu_tlbstate is going away, but yes, native_write_cr4() is the right
interface to use, or rather the cr4_{set,clear,toggle}_bits() things
are.

That gives us control over which CR4 bits are available, and, a possible
means of arbitrating that VMX bit.