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From: Chao Gao <chao.gao@intel.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "Sergey Dyasli" <sergey.dyasli@citrix.com>,
	"Ashok Raj" <ashok.raj@intel.com>, "Wei Liu" <wl@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	xen-devel@lists.xenproject.org,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v9 15/15] microcode: block #NMI handling when loading an ucode
Date: Fri, 30 Aug 2019 14:35:06 +0800
Message-ID: <20190830063506.GD31905@gao-cwp> (raw)
In-Reply-To: <d95a5b28-04c6-dcc6-8b16-fc4ade330c15@suse.com>

On Thu, Aug 29, 2019 at 02:11:10PM +0200, Jan Beulich wrote:
>On 27.08.2019 06:52, Chao Gao wrote:
>> On Mon, Aug 26, 2019 at 04:07:59PM +0800, Chao Gao wrote:
>>> On Fri, Aug 23, 2019 at 09:46:37AM +0100, Sergey Dyasli wrote:
>>>> 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@intel.com>
>>>>> ---
>>>>> 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?
>>>
>>> No special reason. Because the issue we want to address is that slave
>>> threads might go to handle NMI and access MSRs when master thread is
>>> loading ucode. So we only keep slave threads in the NMI handler.
>>>
>>>>
>>>> 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.
>>>
>>> Sorry, I meant using self shorthand to send self-nmi. I tried to use
>>> self shorthand but got APIC error. And I agree that it is better to
>>> make slave thread call self_nmi() itself.
>>>
>>>>
>>>> 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?
>>>
>>> Ashok, what's your opinion on Sergey's approach and his concern?
>> 
>> I talked with Ashok. We think your approach is better. I will follow
>> your approach in v10. It would be much helpful if you post your patch
>> so that I can just rebase it onto other patches.
>
>Doing the actual ucode update inside an NMI handler seems rather risky
>to me. Even if Ashok confirmed it would not be an issue on past and
>current Intel CPUs - what about future ones, or ones from other vendors?

Will confirm these with Ashok.

Thanks
Chao

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Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-19  1:25 [Xen-devel] [PATCH v9 00/15] improve late microcode loading Chao Gao
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 01/15] microcode/intel: extend microcode_update_match() Chao Gao
2019-08-28 15:12   ` Jan Beulich
2019-08-29  7:15     ` Chao Gao
2019-08-29  7:14       ` Jan Beulich
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 02/15] microcode/amd: fix memory leak Chao Gao
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 03/15] microcode/amd: distinguish old and mismatched ucode in microcode_fits() Chao Gao
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 04/15] microcode: introduce a global cache of ucode patch Chao Gao
2019-08-22 11:11   ` Roger Pau Monné
2019-08-28 15:21   ` Jan Beulich
2019-08-29 10:18   ` Jan Beulich
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 05/15] microcode: clean up microcode_resume_cpu Chao Gao
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 06/15] microcode: remove struct ucode_cpu_info Chao Gao
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 07/15] microcode: remove pointless 'cpu' parameter Chao Gao
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 08/15] microcode/amd: call svm_host_osvw_init() in common code Chao Gao
2019-08-22 13:08   ` Roger Pau Monné
2019-08-28 15:26   ` Jan Beulich
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 09/15] microcode: pass a patch pointer to apply_microcode() Chao Gao
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 10/15] microcode: split out apply_microcode() from cpu_request_microcode() Chao Gao
2019-08-22 13:59   ` Roger Pau Monné
2019-08-29 10:06     ` Jan Beulich
2019-08-30  3:22       ` Chao Gao
2019-08-30  7:25         ` Jan Beulich
2019-08-29 10:19   ` Jan Beulich
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 11/15] microcode: unify loading update during CPU resuming and AP wakeup Chao Gao
2019-08-22 14:10   ` Roger Pau Monné
2019-08-22 16:44     ` Chao Gao
2019-08-23  9:09       ` Roger Pau Monné
2019-08-29  7:37         ` Chao Gao
2019-08-29  8:16           ` Roger Pau Monné
2019-08-29 10:26           ` Jan Beulich
2019-08-29 10:29   ` Jan Beulich
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 12/15] microcode: reduce memory allocation and copy when creating a patch Chao Gao
2019-08-23  8:11   ` Roger Pau Monné
2019-08-26  7:03     ` Chao Gao
2019-08-26  8:11       ` Roger Pau Monné
2019-08-29 10:47   ` Jan Beulich
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 13/15] x86/microcode: Synchronize late microcode loading Chao Gao
2019-08-19 10:27   ` Sergey Dyasli
2019-08-19 14:49     ` Chao Gao
2019-08-29 12:06   ` Jan Beulich
2019-08-30  3:30     ` Chao Gao
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 14/15] microcode: remove microcode_update_lock Chao Gao
2019-08-19  1:25 ` [Xen-devel] [PATCH v9 15/15] microcode: block #NMI handling when loading an ucode Chao Gao
2019-08-23  8:46   ` Sergey Dyasli
2019-08-26  8:07     ` Chao Gao
2019-08-27  4:52       ` Chao Gao
2019-08-28  8:52         ` Sergey Dyasli
2019-08-29 12:11         ` Jan Beulich
2019-08-30  6:35           ` Chao Gao [this message]
2019-09-09  5:52             ` Chao Gao
2019-09-09  6:16               ` Jan Beulich
2019-08-29 12:22   ` Jan Beulich
2019-08-30  6:33     ` Chao Gao
2019-08-30  7:30       ` Jan Beulich
2019-08-22  7:51 ` [Xen-devel] [PATCH v9 00/15] improve late microcode loading Sergey Dyasli
2019-08-22 15:39   ` Chao Gao

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