x86,kvm,vmx: Preserve CR4 across VM entry

CR4 isn't constant; at least the TSD and PCE bits can vary.

TBH, treating CR0 and CR3 as constant scares me a bit, too, but it looks
like it's correct.

This adds a branch and a read from cr4 to each vm entry.  Because it is
extremely likely that consecutive entries into the same vcpu will have
the same host cr4 value, this fixes up the vmcs instead of restoring cr4
after the fact.  A subsequent patch will add a kernel-wide cr4 shadow,
reducing the overhead in the common case to just two memory reads and a
branch.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
Cc: Petr Matousek <pmatouse@redhat.com>
Cc: Gleb Natapov <gleb@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
{
	unsigned long hw_cr4 = cr4 | (to_vmx(vcpu)->rmode.vm86_active ?
		    KVM_RMODE_VM_CR4_ALWAYS_ON : KVM_PMODE_VM_CR4_ALWAYS_ON);

	if (cr4 & X86_CR4_VMXE) {
		/*
		 * To use VMXON (and later other VMX instructions), a guest
		 * must first be able to turn on cr4.VMXE (see handle_vmon()).
		 * So basically the check on whether to allow nested VMX
		 * is here.
		 */
		if (!nested_vmx_allowed(vcpu))
			return 1;
	}
	if (to_vmx(vcpu)->nested.vmxon &&
	    ((cr4 & VMXON_CR4_ALWAYSON) != VMXON_CR4_ALWAYSON))
		return 1;

	vcpu->arch.cr4 = cr4;
	if (enable_ept) {
		if (!is_paging(vcpu)) {
			hw_cr4 &= ~X86_CR4_PAE;
			hw_cr4 |= X86_CR4_PSE;
			/*
			 * SMEP/SMAP is disabled if CPU is in non-paging mode
			 * in hardware. However KVM always uses paging mode to
			 * emulate guest non-paging mode with TDP.
			 * To emulate this behavior, SMEP/SMAP needs to be
			 * manually disabled when guest switches to non-paging
			 * mode.
			 */
			hw_cr4 &= ~(X86_CR4_SMEP | X86_CR4_SMAP);
		} else if (!(cr4 & X86_CR4_PAE)) {
			hw_cr4 &= ~X86_CR4_PAE;
		}
	}

	vmcs_writel(CR4_READ_SHADOW, cr4);
	vmcs_writel(GUEST_CR4, hw_cr4);
	return 0;
}

static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
{
	unsigned long hw_cr4 = cr4 | (to_vmx(vcpu)->rmode.vm86_active ?
		    KVM_RMODE_VM_CR4_ALWAYS_ON : KVM_PMODE_VM_CR4_ALWAYS_ON);

	if (cr4 & X86_CR4_VMXE) {
		/*
		 * To use VMXON (and later other VMX instructions), a guest
		 * must first be able to turn on cr4.VMXE (see handle_vmon()).
		 * So basically the check on whether to allow nested VMX
		 * is here.
		 */
		if (!nested_vmx_allowed(vcpu))
			return 1;
	}
	if (to_vmx(vcpu)->nested.vmxon &&
	    ((cr4 & VMXON_CR4_ALWAYSON) != VMXON_CR4_ALWAYSON))
		return 1;

	vcpu->arch.cr4 = cr4;
	if (enable_ept) {
		if (!is_paging(vcpu)) {
			hw_cr4 &= ~X86_CR4_PAE;
			hw_cr4 |= X86_CR4_PSE;
			/*
			 * SMEP/SMAP is disabled if CPU is in non-paging mode
			 * in hardware. However KVM always uses paging mode to
			 * emulate guest non-paging mode with TDP.
			 * To emulate this behavior, SMEP/SMAP needs to be
			 * manually disabled when guest switches to non-paging
			 * mode.
			 */
			hw_cr4 &= ~(X86_CR4_SMEP | X86_CR4_SMAP);
		} else if (!(cr4 & X86_CR4_PAE)) {
			hw_cr4 &= ~X86_CR4_PAE;
		}
	}

	vmcs_writel(CR4_READ_SHADOW, cr4);
	vmcs_writel(GUEST_CR4, hw_cr4);
	return 0;
}
