[xen stable-4.18] x86/emul: Fix misaligned IO breakpoint behaviour in PV guests

[patchbot@xxxxxxx]

commit d446b472cc5051c2ef4eaf5f895d96d7a458c8a1
Author:     Matthew Barnes <matthew.barnes@xxxxxxxxx>
AuthorDate: Thu Aug 8 13:53:40 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Aug 8 13:53:40 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 f101510a1b..aa11ecadaa 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -346,6 +346,8 @@ 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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