Re: [PATCH -v2 1/2] x86, microcode: Sanitize per-cpu microcode reloading interface

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2012-06-21 at 14:07 +0200, Borislav Petkov wrote:
> From: Borislav Petkov <borislav.petkov@amd.com>
> 
> Microcode reloading in a per-core manner is a very bad idea for both
> major x86 vendors. And the thing is, we have such interface with which
> we can end up with different microcode versions applied on different
> cores of an otherwise homogeneous wrt (family,model,stepping) system.
> 
> So turn off the possibility of doing that per core and allow it only
> system-wide.
> 
> This is a minimal fix which we'd like to see in stable too thus the
> more-or-less arbitrary decision to allow system-wide reloading only on
> the BSP:
> 
> $ echo 1 > /sys/devices/system/cpu/cpu0/microcode/reload
> ...
> 
> and disable the interface on the other cores:
> 
> $ echo 1 > /sys/devices/system/cpu/cpu23/microcode/reload
> -bash: echo: write error: Invalid argument
> 
> Also, allowing the reload only from one CPU (the BSP in
> that case) doesn't allow the reload procedure to degenerate
> into an O(n^2) deal when triggering reloads from all
> /sys/devices/system/cpu/cpuX/microcode/reload sysfs nodes
> simultaneously.
> 
> A more generic fix will follow.
> 
> Cc: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Acked-by: Peter Zijlstra <peterz@infradead.org>