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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel