KVM: x86: Virtualize LAM for supervisor pointer

Add support to allow guests to set the new CR4 control bit for LAM and add
implementation to get untagged address for supervisor pointers.

LAM modifies the canonicality check applied to 64-bit linear addresses for
data accesses, allowing software to use of the untranslated address bits for
metadata and masks the metadata bits before using them as linear addresses
to access memory. LAM uses CR4.LAM_SUP (bit 28) to configure and enable LAM
for supervisor pointers. It also changes VMENTER to allow the bit to be set
in VMCS's HOST_CR4 and GUEST_CR4 to support virtualization. Note CR4.LAM_SUP
is allowed to be set even not in 64-bit mode, but it will not take effect
since LAM only applies to 64-bit linear addresses.

Move CR4.LAM_SUP out of CR4_RESERVED_BITS, its reservation depends on vcpu
supporting LAM or not. Leave it intercepted to prevent guest from setting
the bit if LAM is not exposed to guest as well as to avoid vmread every time
when KVM fetches its value, with the expectation that guest won't toggle the
bit frequently.

Set CR4.LAM_SUP bit in the emulated IA32_VMX_CR4_FIXED1 MSR for guests to
allow guests to enable LAM for supervisor pointers in nested VMX operation.

Hardware is not required to do TLB flush when CR4.LAM_SUP toggled, KVM
doesn't need to emulate TLB flush based on it.  There's no other features
or vmx_exec_controls connection, and no other code needed in
{kvm,vmx}_set_cr4().

Skip address untag for instruction fetches (which includes branch targets),
operand of INVLPG instructions, and implicit system accesses, all of which
are not subject to untagging.  Note, get_untagged_addr() isn't invoked for
implicit system accesses as there is no reason to do so, but check the
flag anyways for documentation purposes.

Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
Co-developed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Tested-by: Xuelian Guo <xuelian.guo@intel.com>
Link: https://lore.kernel.org/r/20230913124227.12574-11-binbin.wu@linux.intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 38 insertions, 1 deletions

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d7ff06d77720..3bbe5a9362a9 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7673,6 +7673,9 @@ static void nested_vmx_cr_fixed1_bits_update(struct kvm_vcpu *vcpu)
 	cr4_fixed1_update(X86_CR4_UMIP,       ecx, feature_bit(UMIP));
 	cr4_fixed1_update(X86_CR4_LA57,       ecx, feature_bit(LA57));
 
+	entry = kvm_find_cpuid_entry_index(vcpu, 0x7, 1);
+	cr4_fixed1_update(X86_CR4_LAM_SUP,    eax, feature_bit(LAM));
+
 #undef cr4_fixed1_update
 }
 
@@ -8205,9 +8208,43 @@ static void vmx_vm_destroy(struct kvm *kvm)
 	free_pages((unsigned long)kvm_vmx->pid_table, vmx_get_pid_table_order(kvm));
 }
 
+/*
+ * Note, the SDM states that the linear address is masked *after* the modified
+ * canonicality check, whereas KVM masks (untags) the address and then performs
+ * a "normal" canonicality check.  Functionally, the two methods are identical,
+ * and when the masking occurs relative to the canonicality check isn't visible
+ * to software, i.e. KVM's behavior doesn't violate the SDM.
+ */
 gva_t vmx_get_untagged_addr(struct kvm_vcpu *vcpu, gva_t gva, unsigned int flags)
 {
-	return gva;
+	int lam_bit;
+
+	if (flags & (X86EMUL_F_FETCH | X86EMUL_F_IMPLICIT | X86EMUL_F_INVLPG))
+		return gva;
+
+	if (!is_64_bit_mode(vcpu))
+		return gva;
+
+	/*
+	 * Bit 63 determines if the address should be treated as user address
+	 * or a supervisor address.
+	 */
+	if (!(gva & BIT_ULL(63))) {
+		/* KVM doesn't yet virtualize LAM_U{48,57}. */
+		return gva;
+	} else {
+		if (!kvm_is_cr4_bit_set(vcpu, X86_CR4_LAM_SUP))
+			return gva;
+
+		lam_bit = kvm_is_cr4_bit_set(vcpu, X86_CR4_LA57) ? 56 : 47;
+	}
+
+	/*
+	 * Untag the address by sign-extending the lam_bit, but NOT to bit 63.
+	 * Bit 63 is retained from the raw virtual address so that untagging
+	 * doesn't change a user access to a supervisor access, and vice versa.
+	 */
+	return (sign_extend64(gva, lam_bit) & ~BIT_ULL(63)) | (gva & BIT_ULL(63));
 }
 
 static struct kvm_x86_ops vmx_x86_ops __initdata = {