x86/HVM: use fixed TSC value when saving or restoring domain

When a domain is saved each VCPU's TSC value needs to be preserved. To get it we
use hvm_get_guest_tsc(). This routine (either itself or via get_s_time() which
it may call) calculates VCPU's TSC based on current host's TSC value (by doing a
rdtscll()). Since this is performed for each VCPU separately we end up with
un-synchronized TSCs.

Similarly, during a restore each VCPU is assigned its TSC based on host's current
tick, causing virtual TSCs to diverge further.

With this, we can easily get into situation where a guest may see time going
backwards.

Instead of reading new TSC value for each VCPU when saving/restoring it we should
use the same value across all VCPUs.

Reported-by: Philippe Coquard <philippe.coquard@mpsa.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/hvm.c

@@ -258,16 +258,20 @@ int hvm_set_guest_pat(struct vcpu *v, u64 guest_pat)
     return 1;
 }
 
-void hvm_set_guest_tsc(struct vcpu *v, u64 guest_tsc)
+void hvm_set_guest_tsc_fixed(struct vcpu *v, u64 guest_tsc, u64 at_tsc)
 {
     uint64_t tsc;
     uint64_t delta_tsc;
 
     if ( v->domain->arch.vtsc )
     {
-        tsc = hvm_get_guest_time(v);
+        tsc = hvm_get_guest_time_fixed(v, at_tsc);
         tsc = gtime_to_gtsc(v->domain, tsc);
     }
+    else if ( at_tsc )
+    {
+        tsc = at_tsc;
+    }
     else
     {
         rdtscll(tsc);
@@ -278,7 +282,7 @@ void hvm_set_guest_tsc(struct vcpu *v, u64 guest_tsc)
                           - v->arch.hvm_vcpu.cache_tsc_offset;
     v->arch.hvm_vcpu.cache_tsc_offset = delta_tsc;
 
-    hvm_funcs.set_tsc_offset(v, v->arch.hvm_vcpu.cache_tsc_offset, 0);
+    hvm_funcs.set_tsc_offset(v, v->arch.hvm_vcpu.cache_tsc_offset, at_tsc);
 }
 
 void hvm_set_guest_tsc_adjust(struct vcpu *v, u64 tsc_adjust)

xen/arch/x86/hvm/save.c

@@ -34,6 +34,9 @@ void arch_hvm_save(struct domain *d, struct hvm_save_header *hdr)
 
     /* Save guest's preferred TSC. */
     hdr->gtsc_khz = d->arch.tsc_khz;
+
+    /* Time when saving started */
+    rdtscll(d->arch.hvm_domain.sync_tsc);
 }
 
 int arch_hvm_load(struct domain *d, struct hvm_save_header *hdr)
@@ -67,6 +70,9 @@ int arch_hvm_load(struct domain *d, struct hvm_save_header *hdr)
     if ( d->arch.vtsc )
         hvm_set_rdtsc_exiting(d, 1);
 
+    /* Time when restore started  */
+    rdtscll(d->arch.hvm_domain.sync_tsc);
+
     /* VGA state is not saved/restored, so we nobble the cache. */
     d->arch.hvm_domain.stdvga.cache = 0;
 

xen/arch/x86/hvm/svm/svm.c

@@ -318,7 +318,8 @@ static void svm_save_cpu_state(struct vcpu *v, struct hvm_hw_cpu *data)
     data->msr_efer         = v->arch.hvm_vcpu.guest_efer;
     data->msr_flags        = -1ULL;
 
-    data->tsc = hvm_get_guest_tsc(v);
+    data->tsc = hvm_get_guest_tsc_fixed(v,
+                                        v->domain->arch.hvm_domain.sync_tsc);
 }
 
 
@@ -334,7 +335,7 @@ static void svm_load_cpu_state(struct vcpu *v, struct hvm_hw_cpu *data)
     v->arch.hvm_vcpu.guest_efer = data->msr_efer;
     svm_update_guest_efer(v);
 
-    hvm_set_guest_tsc(v, data->tsc);
+    hvm_set_guest_tsc_fixed(v, data->tsc, v->domain->arch.hvm_domain.sync_tsc);
 }
 
 static void svm_save_vmcb_ctxt(struct vcpu *v, struct hvm_hw_cpu *ctxt)

xen/arch/x86/hvm/vmx/vmx.c

@@ -540,7 +540,8 @@ static void vmx_save_cpu_state(struct vcpu *v, struct hvm_hw_cpu *data)
     data->msr_star         = guest_state->msrs[VMX_INDEX_MSR_STAR];
     data->msr_syscall_mask = guest_state->msrs[VMX_INDEX_MSR_SYSCALL_MASK];
 
-    data->tsc = hvm_get_guest_tsc(v);
+    data->tsc = hvm_get_guest_tsc_fixed(v,
+                                        v->domain->arch.hvm_domain.sync_tsc);
 }
 
 static void vmx_load_cpu_state(struct vcpu *v, struct hvm_hw_cpu *data)
@@ -556,7 +557,7 @@ static void vmx_load_cpu_state(struct vcpu *v, struct hvm_hw_cpu *data)
     v->arch.hvm_vmx.cstar     = data->msr_cstar;
     v->arch.hvm_vmx.shadow_gs = data->shadow_gs;
 
-    hvm_set_guest_tsc(v, data->tsc);
+    hvm_set_guest_tsc_fixed(v, data->tsc, v->domain->arch.hvm_domain.sync_tsc);
 }
 
 

xen/include/asm-x86/hvm/hvm.h

@@ -232,7 +232,8 @@ bool_t hvm_send_assist_req(struct vcpu *v);
 void hvm_get_guest_pat(struct vcpu *v, u64 *guest_pat);
 int hvm_set_guest_pat(struct vcpu *v, u64 guest_pat);
 
-void hvm_set_guest_tsc(struct vcpu *v, u64 guest_tsc);
+void hvm_set_guest_tsc_fixed(struct vcpu *v, u64 guest_tsc, u64 at_tsc);
+#define hvm_set_guest_tsc(v, t) hvm_set_guest_tsc_fixed(v, t, 0)
 u64 hvm_get_guest_tsc_fixed(struct vcpu *v, u64 at_tsc);
 #define hvm_get_guest_tsc(v) hvm_get_guest_tsc_fixed(v, 0)