[xen master] xen/arm64: head: Add missing isb in setup_fixmap()

[patchbot@xxxxxxx]

commit d29a767d045b10b5971e93a1e8a7ee4a08d8601f
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Tue Jul 4 19:59:06 2023 +0100
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Tue Jul 4 20:07:47 2023 +0100

    xen/arm64: head: Add missing isb in setup_fixmap()
    
    On older version of the Arm Arm (ARM DDI 0487E.a, B2-125) there were
    the following paragraph:
    
    "DMB and DSB instructions affect reads and writes to the memory system
    generated by Load/Store instructions and data or unified cache
    maintenance instructions being executed by the PE. Instruction fetches
    or accesses caused by a hardware translation table access are not
    explicit accesses."
    
    Newer revision (e.g. ARM DDI 0487J.a) doesn't have the second sentence
    (it might be somewhere else in the Arm Arm). But the interpretation is
    not much different.
    
    In setup_fixmap(), we write the fixmap area and may be used soon after,
    for instance, to write to the UART. IOW, there could be hardware
    translation table access. So we need to ensure the 'dsb' has completed
    before continuing. Therefore add an 'isb'.
    
    Fixes: 2b11c3646105 ("xen/arm64: head: Remove 1:1 mapping as soon as it is 
not used")
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Henry Wang <Henry.Wang@xxxxxxx>
    Reviewed-by: Luca Fancellu <luca.fancellu@xxxxxxx>
    Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
    Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
---
 xen/arch/arm/arm64/head.S | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index ad9e0ba87a..c09d0b3a1a 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -812,6 +812,11 @@ setup_fixmap:
         create_table_entry boot_second, xen_fixmap, x0, 2, x1, x2, x3
         /* Ensure any page table updates made above have occurred. */
         dsb   nshst
+        /*
+         * The fixmap area will be used soon after. So ensure no hardware
+         * translation happens before the dsb completes.
+         */
+        isb
 
         ret
 ENDPROC(setup_fixmap)
--
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