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SubjectRe: [PATCH 2/2] x86/microcode: Fix CPU synchronization routine
+ Arjan.

On Thu, Mar 15, 2018 at 01:01:32AM -0300, Henrique de Moraes Holschuh wrote:
> A reasonably well-known paper on intel microcode updates[1] profiled
> that very well, years ago (2013). The information about a linear
> increase in update time versus update size comes from that paper (I did
> not attempt to reproduce his findings, though).


If I read this paper correctly:

it is injecting faults and attempting to manipulate some size field -
I'm guessing the encrypted data size. And I'm also guessing that if you
manipulate that size, it would simply take a lot longer to attempt to
decrypt and verify that it is broken microcode and reject it. So it is
not actually a real update - it is just taking a lot longer to reject

Now, I'm talking about genuine microcode updates. And that paper also
claims that they take thousands of cycles.

Now let's look at your previous, hm, "statement":

> Intel takes anything from twenty thousand cycles to several *million*
> cycles per core, proportional to microcode update size.

So you took a fault injection measurement out of context to claim that
*regular* microcode updates take millions of cycles.

So you had to say something - doesn't matter if it is apples and oranges
- as long as it is dramatic. Fuck the truth.

> When I measured my Xeon X5550 workstation doing an early update, the
> Xeon took about 1M cycles for the BSP, and 800k cycles for the APs (see
> below).
> To measure that, as far as I recall I just did a rdtsc right before the

RDTSC gets executed speculatively, so you need barriers around it. I
hope you added them.

> wrmsr, and another right after, and stashed the result somewhere to be
> able to print it out later in the BSP's case. I repeated the process
> (by rebooting) a few times. There was a *lot* of variation, but not
> enough to get it wrong by an order of magnitude.
> I am surprised that this would be news to you, though. It is not like I
> have been quiet about how expensive these updates are on Intel over the
> past years every time I sent you a patch related to this...

Frankly, I don't take you seriously. Even less so after this.


Good mailing practices for 400: avoid top-posting and trim the reply.

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