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Re: [Xen-devel] [PATCH v9 15/15] microcode: block #NMI handling when loading an ucode
- To: Chao Gao <chao.gao@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
- Date: Fri, 23 Aug 2019 09:46:37 +0100
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- Cc: "sergey.dyasli@xxxxxxxxxx >> Sergey Dyasli" <sergey.dyasli@xxxxxxxxxx>, Ashok Raj <ashok.raj@xxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 23 Aug 2019 08:46:58 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 19/08/2019 02:25, Chao Gao wrote:
> register an nmi callback. And this callback does busy-loop on threads
> which are waiting for loading completion. Control threads send NMI to
> slave threads to prevent NMI acceptance during ucode loading.
>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> ---
> Changes in v9:
> - control threads send NMI to all other threads. Slave threads will
> stay in the NMI handling to prevent NMI acceptance during ucode
> loading. Note that self-nmi is invalid according to SDM.
To me this looks like a half-measure: why keep only slave threads in
the NMI handler, when master threads can update the microcode from
inside the NMI handler as well?
You mention that self-nmi is invalid, but Xen has self_nmi() which is
used for apply_alternatives() during boot, so can be trusted to work.
I experimented a bit with the following approach: after loading_state
becomes LOADING_CALLIN, each cpu issues a self_nmi() and rendezvous
via cpu_callin_map into LOADING_ENTER to do a ucode update directly in
the NMI handler. And it seems to work.
Separate question is about the safety of this approach: can we be sure
that a ucode update would not reset the status of the NMI latch? I.e.
can it cause another NMI to be delivered while Xen already handles one?
> - s/rep_nop/cpu_relax
> - remove debug message in microcode_nmi_callback(). Printing debug
> message would take long times and control thread may timeout.
> - rebase and fix conflicts
Thanks,
Sergey
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