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Date: Thu, 21 Feb 2019 12:04:44 +0100
From: Peter Zijlstra <peterz@infradead.org>
Subject: [MODERATED] Re: [patch V2 09/10] MDS basics+ 9
Message-ID: <20190221110444.GD32477@hirez.programming.kicks-ass.net>
References: <20190220150753.665964899@linutronix.de>
 <20190220151400.787843957@linutronix.de>
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To: speck@linutronix.de
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On Wed, Feb 20, 2019 at 04:08:02PM +0100, speck for Thomas Gleixner wrote:
>  static inline void mds_user_clear_cpu_buffers(void)
>  {
> +	if (static_branch_likely(&mds_user_clear_cond)) {
> +		if (__this_cpu_read(mds_cond_clear)) {
> +			__this_cpu_write(mds_cond_clear, 0);
> +			mds_clear_cpu_buffers();
> +		}
> +	}
>  	if (static_branch_likely(&mds_user_clear_always))
>  		mds_clear_cpu_buffers();
>  }

Results in:

998:   e9 0b 00 00 00          jmpq   9a8 <prepare_exit_to_usermode+0x58>
99d:   65 8b 05 00 00 00 00    mov    %gs:0x0(%rip),%eax        # 9a4 <prepare_exit_to_usermode+0x54>
9a0: R_X86_64_PC32      mds_cond_clear-0x4
9a4:   85 c0                   test   %eax,%eax
9a6:   75 0f                   jne    9b7 <prepare_exit_to_usermode+0x67>
9a8:   e9 07 00 00 00          jmpq   9b4 <prepare_exit_to_usermode+0x64>
9ad:   0f 00 2d 00 00 00 00    verw   0x0(%rip)        # 9b4 <prepare_exit_to_usermode+0x64>
9b0: R_X86_64_PC32      .rodata-0x4
9b4:   5b                      pop    %rbx
9b5:   5d                      pop    %rbp
9b6:   c3                      retq
9b7:   65 c7 05 00 00 00 00    movl   $0x0,%gs:0x0(%rip)        # 9c2 <prepare_exit_to_usermode+0x72>
9be:   00 00 00 00
9ba: R_X86_64_PC32      mds_cond_clear-0x8
9c2:   0f 00 2d 00 00 00 00    verw   0x0(%rip)        # 9c9 <prepare_exit_to_usermode+0x79>
9c5: R_X86_64_PC32      .rodata-0x4
9c9:   eb dd                   jmp    9a8 <prepare_exit_to_usermode+0x58>


static inline void mds_user_clear_cpu_buffers(void)
{
       if (!static_branch_likely(&mds_user_clear_always))
               return;

       if (static_branch_likely(&mds_user_clear_cond)) {
               if (!__this_cpu_read(mds_cond_clear))
                       return;

               __this_cpu_write(mds_cond_clear, 0);
       }

       mds_clear_cpu_buffers();
}

results in:

998:   e9 22 00 00 00          jmpq   9bf <prepare_exit_to_usermode+0x6f>
99d:   e9 16 00 00 00          jmpq   9b8 <prepare_exit_to_usermode+0x68>
9a2:   65 8b 05 00 00 00 00    mov    %gs:0x0(%rip),%eax        # 9a9 <prepare_exit_to_usermode+0x59>
9a5: R_X86_64_PC32      mds_cond_clear-0x4
9a9:   85 c0                   test   %eax,%eax
9ab:   74 12                   je     9bf <prepare_exit_to_usermode+0x6f>
9ad:   65 c7 05 00 00 00 00    movl   $0x0,%gs:0x0(%rip)        # 9b8 <prepare_exit_to_usermode+0x68>
9b4:   00 00 00 00
9b0: R_X86_64_PC32      mds_cond_clear-0x8
9b8:   0f 00 2d 00 00 00 00    verw   0x0(%rip)        # 9bf <prepare_exit_to_usermode+0x6f>
9bb: R_X86_64_PC32      .rodata-0x4
9bf:


The only 'downside' is that means @mds_user_clear_always is now a
misnomer (and needs be enabled for cond too).