KVM: x86: Preserve guest's CR0.CD/NW on INIT

Preserve CR0.CD and CR0.NW on INIT instead of forcing them to '1', as
defined by both Intel's SDM and AMD's APM.

Note, current versions of Intel's SDM are very poorly written with
respect to INIT behavior.  Table 9-1. "IA-32 and Intel 64 Processor
States Following Power-up, Reset, or INIT" quite clearly lists power-up,
RESET, _and_ INIT as setting CR0=60000010H, i.e. CD/NW=1.  But the SDM
then attempts to qualify CD/NW behavior in a footnote:

  2. The CD and NW flags are unchanged, bit 4 is set to 1, all other bits
     are cleared.

Presumably that footnote is only meant for INIT, as the RESET case and
especially the power-up case are rather non-sensical.  Another footnote
all but confirms that:

  6. Internal caches are invalid after power-up and RESET, but left
     unchanged with an INIT.

Bare metal testing shows that CD/NW are indeed preserved on INIT (someone
else can hack their BIOS to check RESET and power-up :-D).

Reported-by: Reiji Watanabe <reijiw@google.com>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210713163324.627647-47-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -10792,6 +10792,7 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
 void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 {
 	unsigned long old_cr0 = kvm_read_cr0(vcpu);
+	unsigned long new_cr0;
 	u32 eax, dummy;
 
 	kvm_lapic_reset(vcpu, init_event);
@@ -10878,7 +10879,18 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 	kvm_set_rflags(vcpu, X86_EFLAGS_FIXED);
 	kvm_rip_write(vcpu, 0xfff0);
 
-	static_call(kvm_x86_set_cr0)(vcpu, X86_CR0_NW | X86_CR0_CD | X86_CR0_ET);
+	/*
+	 * CR0.CD/NW are set on RESET, preserved on INIT.  Note, some versions
+	 * of Intel's SDM list CD/NW as being set on INIT, but they contradict
+	 * (or qualify) that with a footnote stating that CD/NW are preserved.
+	 */
+	new_cr0 = X86_CR0_ET;
+	if (init_event)
+		new_cr0 |= (old_cr0 & (X86_CR0_NW | X86_CR0_CD));
+	else
+		new_cr0 |= X86_CR0_NW | X86_CR0_CD;
+
+	static_call(kvm_x86_set_cr0)(vcpu, new_cr0);
 	static_call(kvm_x86_set_cr4)(vcpu, 0);
 	static_call(kvm_x86_set_efer)(vcpu, 0);
 	static_call(kvm_x86_update_exception_bitmap)(vcpu);