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SubjectRe: [PATCH 0/3] x86/microcode: support for microcode update in Xen dom0
On Wed, Oct 12, 2011 at 05:31:50PM -0400, H. Peter Anvin wrote:
> On 10/12/2011 01:40 PM, Konrad Rzeszutek Wilk wrote:
> > Why is it paramount to do it as early as possible? As in, even doing
> > it before Linux kernel is invoked is preferred than during initrd runtime?
> It is paramount to do it as early as possible *because the CPU is
> broken*. That's why there is a microcode update at all. It is
> *supposed* to be installed by BIOS, but for whatever reason it wasn't
> (including user doesn't want to update the BIOS), so the very fact that
> this is done in the OS at all is a bit of a fail.

Oh, that's easy: you know how OEMs support a platform for a while and
then move on to something new and stop updating BIOSen, including ucode
images contained in them?

Well, in that case, the OS is the last possible place where we want to
be able to apply the ucode. And, we want to apply it as _early_ _as_
_possible_. Bootloader doesn't cut it as hpa says below, so next comes
SMP cores bootup code. That's it.

> Doing it in the bootloader is messy because bootloaders typically
> aren't SMP-aware (and really shouldn't need to be), which leaves the
> OS. On native hardware it should ideally be done as early in the
> processor bringup as possible.



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