[xen master] x86/mtrr: avoid system wide rendezvous when setting AP MTRRs

[patchbot@xxxxxxx]

commit abd00b037da5ffa4e8c4508a5df0cd6eabb805a4
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Mon May 13 10:59:25 2024 +0200
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed May 15 19:59:52 2024 +0100

    x86/mtrr: avoid system wide rendezvous when setting AP MTRRs
    
    There's no point in forcing a system wide update of the MTRRs on all 
processors
    when there are no changes to be propagated.  On AP startup it's only the AP
    that needs to write the system wide MTRR values in order to match the rest 
of
    the already online CPUs.
    
    We have occasionally seen the watchdog trigger during `xen-hptool 
cpu-online`
    in one Intel Cascade Lake box with 448 CPUs due to the re-setting of the 
MTRRs
    on all the CPUs in the system.
    
    While there adjust the comment to clarify why the system-wide resetting of 
the
    MTRR registers is not needed for the purposes of mtrr_ap_init().
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Release-acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/cpu/mtrr/main.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/cpu/mtrr/main.c b/xen/arch/x86/cpu/mtrr/main.c
index 90b235f57e..0a44ebbcb0 100644
--- a/xen/arch/x86/cpu/mtrr/main.c
+++ b/xen/arch/x86/cpu/mtrr/main.c
@@ -573,14 +573,15 @@ void mtrr_ap_init(void)
        if (!mtrr_if || hold_mtrr_updates_on_aps)
                return;
        /*
-        * Ideally we should hold mtrr_mutex here to avoid mtrr entries changed,
-        * but this routine will be called in cpu boot time, holding the lock
-        * breaks it. This routine is called in two cases: 1.very earily time
-        * of software resume, when there absolutely isn't mtrr entry changes;
-        * 2.cpu hotadd time. We let mtrr_add/del_page hold cpuhotplug lock to
-        * prevent mtrr entry changes
+        * hold_mtrr_updates_on_aps takes care of preventing unnecessary MTRR
+        * updates when batch starting the CPUs (see
+        * mtrr_aps_sync_{begin,end}()).
+        *
+        * Otherwise just apply the current system wide MTRR values to this AP.
+        * Note this doesn't require synchronization with the other CPUs, as
+        * there are strictly no modifications of the current MTRR values.
         */
-       set_mtrr(~0U, 0, 0, 0);
+       mtrr_set_all();
 }
 
 /**
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