KVM: x86: Do not mark all registers as avail/dirty during RESET/INIT

Do not blindly mark all registers as available+dirty at RESET/INIT, and
instead rely on writes to registers to go through the proper mutators or
to explicitly mark registers as dirty.  INIT in particular does not blindly
overwrite all registers, e.g. select bits in CR0 are preserved across INIT,
thus marking registers available+dirty without first reading the register
from hardware is incorrect.

In practice this is a benign bug as KVM doesn't let the guest control CR0
bits that are preserved across INIT, and all other true registers are
explicitly written during the RESET/INIT flows.  The PDPTRs and EX_INFO
"registers" are not explicitly written, but accessing those values during
RESET/INIT is nonsensical and would be a KVM bug regardless of register
caching.

Fixes: 66f7b72e ("KVM: x86: Make register state after reset conform to specification")
[sean: !!! NOT FOR STABLE !!!]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210921000303.400537-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -4449,6 +4449,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 	kvm_set_cr8(vcpu, 0);
 
 	vmx_segment_cache_clear(vmx);
+	kvm_register_mark_available(vcpu, VCPU_EXREG_SEGMENTS);
 
 	seg_setup(VCPU_SREG_CS);
 	vmcs_write16(GUEST_CS_SELECTOR, 0xf000);

arch/x86/kvm/x86.c

@@ -10859,9 +10859,9 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 		vcpu->arch.xcr0 = XFEATURE_MASK_FP;
 	}
 
+	/* All GPRs except RDX (handled below) are zeroed on RESET/INIT. */
 	memset(vcpu->arch.regs, 0, sizeof(vcpu->arch.regs));
-	vcpu->arch.regs_avail = ~0;
-	vcpu->arch.regs_dirty = ~0;
+	kvm_register_mark_dirty(vcpu, VCPU_REGS_RSP);
 
 	/*
 	 * Fall back to KVM's default Family/Model/Stepping of 0x600 (P6/Athlon)