x86/bugs, KVM: Support the combination of guest and host IBRS

A guest may modify the SPEC_CTRL MSR from the value used by the
kernel. Since the kernel doesn't use IBRS, this means a value of zero is
what is needed in the host.

But the 336996-Speculative-Execution-Side-Channel-Mitigations.pdf refers to
the other bits as reserved so the kernel should respect the boot time
SPEC_CTRL value and use that.

This allows to deal with future extensions to the SPEC_CTRL interface if
any at all.

Note: This uses wrmsrl() instead of native_wrmsl(). I does not make any
difference as paravirt will over-write the callq *0xfff.. with the wrmsrl
assembler code.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/bugs.c

@@ -123,6 +123,24 @@ u64 x86_spec_ctrl_get_default(void)
 }
 EXPORT_SYMBOL_GPL(x86_spec_ctrl_get_default);
 
+void x86_spec_ctrl_set_guest(u64 guest_spec_ctrl)
+{
+	if (!boot_cpu_has(X86_FEATURE_IBRS))
+		return;
+	if (x86_spec_ctrl_base != guest_spec_ctrl)
+		wrmsrl(MSR_IA32_SPEC_CTRL, guest_spec_ctrl);
+}
+EXPORT_SYMBOL_GPL(x86_spec_ctrl_set_guest);
+
+void x86_spec_ctrl_restore_host(u64 guest_spec_ctrl)
+{
+	if (!boot_cpu_has(X86_FEATURE_IBRS))
+		return;
+	if (x86_spec_ctrl_base != guest_spec_ctrl)
+		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+}
+EXPORT_SYMBOL_GPL(x86_spec_ctrl_restore_host);
+
 #ifdef RETPOLINE
 static bool spectre_v2_bad_module;