[PATCH v1] x86/svm: Intercept CR0 writes selectively

Xen does not need to track when the TS or MP bits change so opt to
intercept CR0 writes selectively. Aside from potentially reducing a few
VMEXITs, this fixes a nested virt bug where L1 intercepts CR0_SEL_WRITE
and L0 intercepts CR0_WRITE. The hardware prioritizes CR0_WRITE and so
L1 never sees any CR0 writes.

Since CR0 may now change behind Xen's back, sync it on VMEXIT so that
the emulator sees the correct value.

Signed-off-by: Ross Lagerwall <[email protected]>
---
 xen/arch/x86/hvm/svm/svm.c  | 7 +++++--
 xen/arch/x86/hvm/svm/vmcb.c | 8 +++++---
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 5f5d903d872d..8da879a5af67 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1640,7 +1640,8 @@ static void svm_vmexit_do_cr_access(
 {
     int gp, cr, dir, rc;
 
-    cr = vmcb->exitcode - VMEXIT_CR0_READ;
+    cr = (vmcb->exitcode == VMEXIT_CR0_SEL_WRITE)
+         ? 16 : (vmcb->exitcode - VMEXIT_CR0_READ);
     dir = (cr > 15);
     cr &= 0xf;
     gp = vmcb->ei.mov_cr.gpr;
@@ -2517,6 +2518,7 @@ void asmlinkage svm_vmexit_handler(void)
     hvm_sanitize_regs_fields(
         regs, !(vmcb_get_efer(vmcb) & EFER_LMA) || !(vmcb->cs.l));
 
+    v->arch.hvm.guest_cr[0] = vmcb_get_cr0(vmcb);
     v->arch.hvm.guest_cr[2] = vmcb_get_cr2(vmcb);
     if ( paging_mode_hap(v->domain) )
         v->arch.hvm.guest_cr[3] = v->arch.hvm.hw_cr[3] = vmcb_get_cr3(vmcb);
@@ -2882,7 +2884,8 @@ void asmlinkage svm_vmexit_handler(void)
         break;
 
     case VMEXIT_CR0_READ ... VMEXIT_CR15_READ:
-    case VMEXIT_CR0_WRITE ... VMEXIT_CR15_WRITE:
+    case VMEXIT_CR1_WRITE ... VMEXIT_CR15_WRITE:
+    case VMEXIT_CR0_SEL_WRITE:
         if ( cpu_has_svm_decode && vmcb->ei.mov_cr.mov_insn )
             svm_vmexit_do_cr_access(vmcb, regs);
         else if ( !hvm_emulate_one_insn(x86_insn_is_cr_access, "CR access") )
diff --git a/xen/arch/x86/hvm/svm/vmcb.c b/xen/arch/x86/hvm/svm/vmcb.c
index 975a1eaef806..7ee91937b10c 100644
--- a/xen/arch/x86/hvm/svm/vmcb.c
+++ b/xen/arch/x86/hvm/svm/vmcb.c
@@ -56,7 +56,7 @@ static int construct_vmcb(struct vcpu *v)
         GENERAL1_INTERCEPT_HLT         | GENERAL1_INTERCEPT_INVLPG      |
         GENERAL1_INTERCEPT_INVLPGA     | GENERAL1_INTERCEPT_IOIO_PROT   |
         GENERAL1_INTERCEPT_MSR_PROT    | GENERAL1_INTERCEPT_SHUTDOWN_EVT|
-        GENERAL1_INTERCEPT_TASK_SWITCH;
+        GENERAL1_INTERCEPT_TASK_SWITCH | GENERAL1_INTERCEPT_CR0_SEL_WRITE;
     vmcb->_general2_intercepts =
         GENERAL2_INTERCEPT_VMRUN       | GENERAL2_INTERCEPT_VMMCALL     |
         GENERAL2_INTERCEPT_VMLOAD      | GENERAL2_INTERCEPT_VMSAVE      |
@@ -76,11 +76,13 @@ static int construct_vmcb(struct vcpu *v)
     /* Intercept all debug-register writes. */
     vmcb->_dr_intercepts = ~0u;
 
-    /* Intercept all control-register accesses except for CR2 and CR8. */
+    /* Intercept all control-register accesses except for CR2, CR8 and
+     * CR0 (covered by selective write). */
     vmcb->_cr_intercepts = ~(CR_INTERCEPT_CR2_READ |
                              CR_INTERCEPT_CR2_WRITE |
                              CR_INTERCEPT_CR8_READ |
-                             CR_INTERCEPT_CR8_WRITE);
+                             CR_INTERCEPT_CR8_WRITE |
+                             CR_INTERCEPT_CR0_WRITE);
 
     svm->vmcb_sync_state = vmcb_needs_vmload;
 
-- 
2.53.0