x86/emul: Fix misaligned IO breakpoint behaviour in PV guests

When hardware breakpoints are configured on misaligned IO ports, the
hardware will mask the addresses based on the breakpoint width during
comparison.

For PV guests, misaligned IO breakpoints do not behave the same way, and
therefore yield different results.

This patch tweaks the emulation of IO breakpoints for PV guests such
that they reproduce the same behaviour as hardware.

Fixes: bec9e3205018 ("x86: emulate I/O port access breakpoints")
Signed-off-by: Matthew Barnes <matthew.barnes@cloud.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 08aacc392d86d4c7dbebdb5e664060ae2af72057
master date: 2024-08-08 13:27:50 +0200

diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index e429dfa4f0a9065f3f7b40f15d55d826858d4397..f216c264120595eda87ae2d02b3eedf20f30f24f 100644 (file)
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -358,6 +358,8 @@ static unsigned int check_guest_io_breakpoint(struct vcpu *v,
         case DR_LEN_8: width = 8; break;
         }
 
+        start &= ~(width - 1UL);
+
         if ( (start < (port + len)) && ((start + width) > port) )
             match |= 1u << i;
     }