[xen stable-4.22] x86/amd: Fix inverted condition in zenbleed_use_chickenbit()

[patchbot@xxxxxxx]

commit 26cb5f56dea2660f8fe2755b4b8371a193f17344
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Sep 29 16:45:52 2026 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Oct 13 13:48:17 2026 +0100

    x86/amd: Fix inverted condition in zenbleed_use_chickenbit()
    
    The identified commit inverted the condition under which Xen chooses
    to set the Zenbleed chickenbit.
    
    Undo that, and leave a better comment explaining why.
    
    Found by AI review of the referenced commit.
    
    This is XSA-515 / CVE-2026-79608.
    
    Fixes: d0c75dc4c028 ("x86/amd: Fix race editing DE_CFG")
    Assisted-by: Sashiko + Opus 5.0
    Reported-by: George Dunlap <dunlapg@xxxxxxxxx>
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 0c5ec39d9106e6a06bf153400e8e1e324e457583)
---
 xen/arch/x86/cpu/amd.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index 70783c9a0a..148886fe2e 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -891,7 +891,12 @@ static bool zenbleed_use_chickenbit(void)
         return is_zen2_uarch();
     }
 
-    return (uint8_t)curr_rev >= fixed_rev;
+    /*
+     * Zenbleed can be mitigated by disabling Floating-Point
+     * Mov-Elimination, but this is expensive.  Only do so when we
+     * don't have a fix in microcode.
+     */
+    return (uint8_t)curr_rev < fixed_rev;
 }
 
 void amd_init_de_cfg(const struct cpuinfo_x86 *c)
--
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