[Bug 203571] New: may_use_simd() returns false in kworkers

https://bugzilla.kernel.org/show_bug.cgi?id=203571

            Bug ID: 203571
           Summary: may_use_simd() returns false in kworkers
           Product: Platform Specific/Hardware
           Version: 2.5
    Kernel Version: 4.19
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: PPC-64
          Assignee: platform_ppc-64@kernel-bugs.osdl.org
          Reporter: slandden@gmail.com
        Regression: No

This patch optimizing chacha20 for WireGuard
https://github.com/shawnl/WireGuard/commit/3e02fce92a14cba7b7d1e2733def3a51bec97498

doesn't work because may_use_simd() is always returning false in kworkers. If I
remove the check everything seems to work fine.

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https://bugzilla.kernel.org/show_bug.cgi?id=203571

--- Comment #1 from Shawn Landden (slandden@gmail.com) ---
                                                      ▒
-   11.41%     0.01%  kworker/1:2-mm_  [kernel.vmlinux]            [k]

process_one_work                                                               
         ▒
   - 11.41% process_one_work                                                   
                                                                               
▒
      - 10.28% wg_packet_tx_worker                                             
                                                                               
▒
         - 10.06% wg_socket_send_skb_to_peer                                   
                                                                               
▒
            - 7.38% _raw_read_unlock_bh                                        
                                                                               
▒
               - 7.35% __local_bh_enable_ip                                    
                                                                               
▒
                  - 7.34% do_softirq.part.2                                    
                                                                               
▒
                     - 7.31% do_softirq_own_stack                              
                                                                               
▒
                        + 7.28% call_do_softirq                                
                                                                               
▒
            - 2.45% send4                                                      
                                                                               
▒
               - 1.85% udp_tunnel_xmit_skb                                     
                                                                               
▒
                  - 1.66% iptunnel_xmit                                        
                                                                               
▒
                     - 1.33% ip_local_out                                      
                                                                               
▒
                        - 1.14% ip_output                                      
                                                                               
▒
                           - 1.06% ip_finish_output2                           
                                                                               
▒
                              + 0.99% __dev_queue_xmit                         
                                                                               
▒
      - 1.06% wg_packet_encrypt_worker                                         
                                                                               
▒
         - 1.03% encrypt_packet                                                
                                                                               
▒
            - 0.98% chacha20poly1305_encrypt_sg                                
                                                                               
▒
                 0.59% chacha20

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--- Comment #2 from Shawn Landden (slandden@gmail.com) ---

X86 manages to allow SIMD in interrupts through some very careful code in
arch/x86/kernel/fpu/core.c (starting with irq_fpu_usable())

PowerPC can do the same.

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