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SubjectRe: [PATCH v4] x86/microcode/intel: Blacklist the specific BDW-EP for late loading
On Fri, Dec 29, 2017 at 10:12:34PM +0800, Jia Zhang wrote:
> Thanks for clarifying the terms. What I meant was the microcode update
> has to be launched in BIOS rather than late loading in linux runtime.

Microcode update on those machines can happen in two ways:

* loaded by the BIOS


* loaded early by the kernel from the initrd.

[ There's also the builtin microcode option which happens early too
but I'm mentioning it here only for completeness sake - don't get
confused by it. ]

> But I'm not quite sure whether or not the early-initrd-based loading is
> affected.

Apparently it isn't.

> I used a shared BDW-EP machine with a rev of microcode less than
> 0x0b000021 and observed the late loading is prohibited. I attempted
> to remove the is_blacklisted() and observed a system hang with the
> late loading. Eventually I followed up the erratum and updated the
> microcode through BIOS update.

Yap, you confirmed the erratum. Which is a good data point. :-)

> It is impossible to downgrade the BIOS to recover the old rev of
> microcode for me to have a test with early-initrd-based loading.

Some BIOSes allow that. Depends on the platform.

> Anyway, is_blacklisted() initially was introduced by Borislav to
> prohibit late loading only, so the early-initrd-based loading seems to
> be OK.

Yes, Borislav, that is me. :-)

And yes, the blacklisting is for late loading only. This has a greater
importance for old kernels where the initrd method is not supported yet
and we wanted to prevent machine hangs from microcode update. Thus this
solution which got backported.


ECO tip #101: Trim your mails when you reply.

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