[Xen-devel] [PATCHv2] x86: fix delta calculation in TSC deadline timer emulation

[David Vrabel <david.vrabel@xxxxxxxxxx>]

From: David Vrabel <david.vrabel@xxxxxxxxxx>

In the virtual LAPIC, correct the delta calculation when emulating the
TSC deadline timer.

Without this fix, XenServer (which is based on Xen 4.1) does not work
when running as an HVM guest.  dom0 fails to boot because its timer
interrupts are very delayed (by several minutes in some cases).

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
---
A 4.1.x candidate?

Changes since v1:
- remove unused guest_time variable
---
 xen/arch/x86/hvm/vlapic.c |   12 ++++--------
 1 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/hvm/vlapic.c b/xen/arch/x86/hvm/vlapic.c
index 8401756..abdb556 100644
--- a/xen/arch/x86/hvm/vlapic.c
+++ b/xen/arch/x86/hvm/vlapic.c
@@ -898,7 +898,6 @@ uint64_t  vlapic_tdt_msr_get(struct vlapic *vlapic)
 void vlapic_tdt_msr_set(struct vlapic *vlapic, uint64_t value)
 {
     uint64_t guest_tsc;
-    uint64_t guest_time;
     struct vcpu *v = vlapic_vcpu(vlapic);
 
     /* may need to exclude some other conditions like vlapic->hw.disabled */
@@ -910,12 +909,10 @@ void vlapic_tdt_msr_set(struct vlapic *vlapic, uint64_t 
value)
     
     /* new_value = 0, >0 && <= now, > now */
     guest_tsc = hvm_get_guest_tsc(v);
-    guest_time = hvm_get_guest_time(v);
     if ( value > guest_tsc )
     {
-        uint64_t delta = value - v->arch.hvm_vcpu.cache_tsc_offset;
-        delta = gtsc_to_gtime(v->domain, delta);
-        delta = max_t(s64, delta - guest_time, 0);
+        uint64_t delta = gtsc_to_gtime(v->domain, value - guest_tsc);
+        delta = max_t(s64, delta, 0);
 
         HVM_DBG_LOG(DBG_LEVEL_VLAPIC_TIMER, "delta[0x%016"PRIx64"]", delta);
 
@@ -949,9 +946,8 @@ void vlapic_tdt_msr_set(struct vlapic *vlapic, uint64_t 
value)
 
     HVM_DBG_LOG(DBG_LEVEL_VLAPIC_TIMER,
                 "tdt_msr[0x%016"PRIx64"],"
-                " gtsc[0x%016"PRIx64"],"
-                " gtime[0x%016"PRIx64"]",
-                vlapic->hw.tdt_msr, guest_tsc, guest_time);
+                " gtsc[0x%016"PRIx64"]",
+                vlapic->hw.tdt_msr, guest_tsc);
 }
 
 static int __vlapic_accept_pic_intr(struct vcpu *v)
-- 
1.7.2.5


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