x86/vlapic: Fix handling of writes to APIC_ESR

Xen currently presents APIC_ESR to guests as a simple read/write register.

This is incorrect.  The SDM states:

  The ESR is a write/read register. Before attempt to read from the ESR,
  software should first write to it. (The value written does not affect the
  values read subsequently; only zero may be written in x2APIC mode.) This
  write clears any previously logged errors and updates the ESR with any
  errors detected since the last write to the ESR.

Introduce a new pending_esr field in hvm_hw_lapic.

Update vlapic_error() to accumulate errors here, and extend vlapic_reg_write()
to discard the written value and transfer pending_esr into APIC_ESR.  Reads
are still as before.

Importantly, this means that guests no longer destroys the ESR value it's
looking for in the LVTERR handler when following the SDM instructions.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/vlapic.c

@@ -108,7 +108,7 @@ static void vlapic_error(struct vlapic *vlapic, unsigned int errmask)
     uint32_t esr;
 
     spin_lock_irqsave(&vlapic->esr_lock, flags);
-    esr = vlapic_get_reg(vlapic, APIC_ESR);
+    esr = vlapic->hw.pending_esr;
     if ( (esr & errmask) != errmask )
     {
         uint32_t lvterr = vlapic_get_reg(vlapic, APIC_LVTERR);
@@ -127,7 +127,7 @@ static void vlapic_error(struct vlapic *vlapic, unsigned int errmask)
                  errmask |= APIC_ESR_RECVILL;
         }
 
-        vlapic_set_reg(vlapic, APIC_ESR, esr | errmask);
+        vlapic->hw.pending_esr |= errmask;
 
         if ( inj )
             vlapic_set_irq(vlapic, lvterr & APIC_VECTOR_MASK, 0);
@@ -802,6 +802,19 @@ void vlapic_reg_write(struct vcpu *v, unsigned int reg, uint32_t val)
         vlapic_set_reg(vlapic, APIC_ID, val);
         break;
 
+    case APIC_ESR:
+    {
+        unsigned long flags;
+
+        spin_lock_irqsave(&vlapic->esr_lock, flags);
+        val = vlapic->hw.pending_esr;
+        vlapic->hw.pending_esr = 0;
+        spin_unlock_irqrestore(&vlapic->esr_lock, flags);
+
+        vlapic_set_reg(vlapic, APIC_ESR, val);
+        break;
+    }
+
     case APIC_TASKPRI:
         vlapic_set_reg(vlapic, APIC_TASKPRI, val & 0xff);
         break;

xen/include/public/arch-x86/hvm/save.h

@@ -394,6 +394,7 @@ struct hvm_hw_lapic {
     uint32_t             disabled; /* VLAPIC_xx_DISABLED */
     uint32_t             timer_divisor;
     uint64_t             tdt_msr;
+    uint32_t             pending_esr;
 };
 
 DECLARE_HVM_SAVE_TYPE(LAPIC, 5, struct hvm_hw_lapic);