KVM/VMX: Allow direct access to MSR_IA32_SPEC_CTRL

[ Based on a patch from Ashok Raj <[email protected]> ]

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 <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Darren Kenny <[email protected]>
Reviewed-by: Konrad Rzeszutek Wilk <[email protected]>
Reviewed-by: Jim Mattson <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Jun Nakajima <[email protected]>
Cc: [email protected]
Cc: Dave Hansen <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Asit Mallick <[email protected]>
Cc: Arjan Van De Ven <[email protected]>
Cc: Greg KH <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Ashok Raj <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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