Messages in this thread | | | Date | Wed, 28 Feb 2018 05:26:40 -0800 | From | "Raj, Ashok" <> | Subject | Re: [PATCH 4/7] x86/microcode: Do not upload microcode if CPUs are offline |
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On Wed, Feb 28, 2018 at 10:11:56AM -0300, Henrique de Moraes Holschuh wrote: > > Avoid loading microcode if any of the CPUs are offline, and issue a > > warning. Having different microcode revisions on the system at any time > > is outright dangerous. > > Even if we update that microcode during CPU early bring-up, before we > mark it on-line and start using it? > > AFAIK, late-loading or not, this is what should happen in the current > code: APs that are brought up after a microcode update is loaded (either > by the early or late driver, it doesn't matter) will be always > *early-updated* to the new microcode. > > Is it dangerous to have an offline core at an older microcode revision > than the online cores?
We don't want to leave a system and allow the user to update microcode when some cpus are offline. Remember cpu_offline in linux is only logical offlining.. so the cpu is still in the system.. it can even participate in MCE for e.g. It is very much alive. Its not a question that "Would it not work" but its not worth to leave an open door and being paranoid is best!
> > I am not against the patch, mind you, but I am curious about why it is > supposed to be dangerous if we're updating the CPUs before we start > using them *anyway*. > > Also, if this is really dangerous, does it means safe CPU hotplug isn't > possible? AFAICT, the firmware would have to do it for us, but it > *doesn't* have the up-to-date microcode (*we* had to update it)...
The difference is hot-adding you know you are going to update the current microcode. But leaving a cpu in offline state is leaving it stale for a long time without realizing that you have some stale cores.
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