Re: [PATCH 20/24] KVM/VMX: Allow direct access to MSR_IA32_SPEC_CTRL

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Apr 18, 2018 at 11:18:28AM +0800, Youquan Song wrote:
> From: KarimAllah Ahmed <karahmed@amazon.de>
> 
> (cherry picked from commit d28b387fb74da95d69d2615732f50cceb38e9a4d)
> 
> [ Based on a patch from Ashok Raj <ashok.raj@intel.com> ]
> 
> Add direct access to MSR_IA32_SPEC_CTRL for guests. This is needed for
> guests that will only mitigate Spectre V2 through IBRS+IBPB and will not
> be using a retpoline+IBPB based approach.
> 
> To avoid the overhead of saving and restoring the MSR_IA32_SPEC_CTRL for
> guests that do not actually use the MSR, only start saving and restoring
> when a non-zero is written to it.
> 
> No attempt is made to handle STIBP here, intentionally. Filtering STIBP
> may be added in a future patch, which may require trapping all writes
> if we don't want to pass it through directly to the guest.
> 
> [dwmw2: Clean up CPUID bits, save/restore manually, handle reset]
> 
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Reviewed-by: Jim Mattson <jmattson@google.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: Jun Nakajima <jun.nakajima@intel.com>
> Cc: kvm@vger.kernel.org
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Asit Mallick <asit.k.mallick@intel.com>
> Cc: Arjan Van De Ven <arjan.van.de.ven@intel.com>
> Cc: Greg KH <gregkh@linuxfoundation.org>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Ashok Raj <ashok.raj@intel.com>
> Link: https://lkml.kernel.org/r/1517522386-18410-5-git-send-email-karahmed@amazon.de
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com> [v4.4 backport]
> 
> Conflicts:
> 	arch/x86/kvm/vmx.c

yet again...