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[Xen-devel] [PATCH v3] x86/AMD: flush TLB after ucode update

The increased number of messages (spec_ctrl.c:print_details()) within a
certain time window made me notice some slowness of boot time screen
output. Experimentally I've narrowed the time window to be from
immediately after the early ucode update on the BSP to the PAT write in
cpu_init(), which upon further investigation has an effect because of
the full TLB flush that's implied by that write.

For that reason, as a workaround, flush the TLB of the mapping of the
page that holds the blob. Note that flushing just a single page is
sufficient: As per verify_patch_size() patch size can't exceed 4k, and
the way xmalloc() works the blob can't be crossing a page boundary.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
v3: Use TLB flush instead of PAT write.
v2: Re-base.

--- a/xen/arch/x86/microcode_amd.c
+++ b/xen/arch/x86/microcode_amd.c
@@ -218,6 +218,12 @@ static int apply_microcode(unsigned int
     spin_unlock_irqrestore(&microcode_update_lock, flags);
+    /*
+     * Experimentally this helps with performance issues on at least certain
+     * Fam15 models.
+     */
+    flush_area_local(hdr, FLUSH_TLB_GLOBAL | FLUSH_ORDER(0));
     /* check current patch id and patch's id for match */
     if ( hw_err || (rev != hdr->patch_id) )

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