Re: [Xen-devel] [PATCH v5 8/8] microcode: update microcode on cores in parallel

[Chao Gao <chao.gao@xxxxxxxxx>]

On Tue, Feb 12, 2019 at 06:55:20AM -0700, Jan Beulich wrote:
>>>> On 12.02.19 at 14:25, <roger.pau@xxxxxxxxxx> wrote:
>> On Tue, Feb 12, 2019 at 05:51:41AM -0700, Jan Beulich wrote:
>>> >>> On 28.01.19 at 08:06, <chao.gao@xxxxxxxxx> wrote:
>>> > @@ -314,9 +310,7 @@ static int apply_microcode(unsigned int cpu)
>>> >  
>>> >      mc_intel = patch->data;
>>> >      BUG_ON(!mc_intel);
>>> > -
>>> > -    /* serialize access to the physical write to MSR 0x79 */
>>> > -    spin_lock_irqsave(&microcode_update_lock, flags);
>>> > +    BUG_ON(local_irq_is_enabled());
>>> >  
>>> >      /* write microcode via MSR 0x79 */
>>> >      wrmsrl(MSR_IA32_UCODE_WRITE, (unsigned long)mc_intel->bits);
>>> > @@ -329,7 +323,6 @@ static int apply_microcode(unsigned int cpu)
>>> >      rdmsrl(MSR_IA32_UCODE_REV, msr_content);
>>> >      val[1] = (uint32_t)(msr_content >> 32);
>>> >  
>>> > -    spin_unlock_irqrestore(&microcode_update_lock, flags);
>>> >      if ( val[1] != mc_intel->hdr.rev )
>>> >      {
>>> >          printk(KERN_ERR "microcode: CPU%d update from revision "
>>> 
>>> Am I understanding right that you now rely on upper layers in the
>>> call tree to avoid calling into here in parallel for two hyperthreads
>>> of the same core? I can't see how you avoid this situation during
>>> AP bringup, for example. Did I overlook anything in this regard?
>> 
>> IIRC microcode update is done in the serialized part of AP bringup,
>> before the call to smp_callin, which guarantees serialization.
>
>Hmm, yes, right now it is. But I'd call this "happens to be that way"
>rather than "is guaranteed to be that way" - prior to commit
>f97838bbd9 it did happen later.

How about employing another lock, "early_ucode_update_lock", to
guarantee serialization.

In particular, early_microcode_update_cpu() and microcode_resume_cpu()
will acquire this lock before ucode update.

Thanks
Chao

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