[xen staging-4.17] x86/emul: Fix misaligned IO breakpoint behaviour in PV guests

[patchbot@xxxxxxx]

commit 72db7ba7dbd1287101b9533cd7e4c877f31a0764
Author:     Matthew Barnes <matthew.barnes@xxxxxxxxx>
AuthorDate: Thu Aug 8 13:57:43 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Aug 8 13:57:43 2024 +0200

    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@xxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 08aacc392d86d4c7dbebdb5e664060ae2af72057
    master date: 2024-08-08 13:27:50 +0200
---
 xen/arch/x86/pv/emul-priv-op.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index e429dfa4f0..f216c26412 100644
--- 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;
     }
--
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