[PATCH] x86/HVM: correct partial HPET_STATUS write emulation

[Jan Beulich <jbeulich@xxxxxxxx>]

For partial writes the non-written parts of registers are folded into
the full 64-bit value from what they're presently set to. That's wrong
to do though when the behavior is write-1-to-clear: Writes not
including to low 3 bits would unconditionally clear all ISR bits which
are presently set. Re-calculate the value to use.

Fixes: be07023be115 ("x86/vhpet: add support for level triggered interrupts")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Alternatively we could also suppress the folding in of read bits, but
that looked to me to end up in a more intrusive change.

--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -404,7 +404,8 @@ static int cf_check hpet_write(
         break;
 
     case HPET_STATUS:
-        /* write 1 to clear. */
+        /* Write 1 to clear. Therefore don't use new_val directly here. */
+        new_val = val << ((addr & 7) * 8);
         while ( new_val )
         {
             bool active;