Re: [Xen-devel] [PATCH v10 00/16] improve late microcode loading

From: Jan Beulich <jbeulich@suse.com>
To: Chao Gao <chao.gao@intel.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>,
	"Suravee Suthikulpanit" <suravee.suthikulpanit@amd.com>,
	xen-devel@lists.xenproject.org,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v10 00/16] improve late microcode loading
Date: Tue, 17 Sep 2019 09:11:44 +0200	[thread overview]
Message-ID: <109a49e9-eb11-ec5d-2de3-18cceaa6d8b3@suse.com> (raw)
In-Reply-To: <20190917070922.GA10190@gao-cwp>

On 17.09.2019 09:09, Chao Gao wrote:
> On Fri, Sep 13, 2019 at 10:47:36AM +0200, Jan Beulich wrote:
>> On 12.09.2019 09:22, Chao Gao wrote:
>>> This series includes below changes:
>>>  1. Patch 1-11: introduce a global microcode cache and some cleanup
>>>  2. Patch 12: synchronize late microcode loading
>>>  3. Patch 13: support parallel microcodes update on different cores
>>>  4. Patch 14: block #NMI handling during microcode loading
>>>  5. Patch 15: disable late ucode loading due to BDF90
>>>  6. Patch 16: call wbinvd() conditionally
>>
>> I don't know why it didn't occur to me earlier, but what about
>> parked / offlined CPUs? They'll have their ucode updated when they
>> get brought back online, but until then their ucode will disagree
>> with that of the online CPUs. For truly offline CPUs this may be
>> fine, but parked ones should probably be updated, perhaps via the
>> same approach as used when C-state data becomes available (see
>> set_cx_pminfo())?
> 
> Yes. It provides a means to wake up the parked CPU and a chance to run
> some code (like loading ucode). But parked CPUs are cleared from
> sibling info and cpu_online_map (see __cpu_disable()). If parallel
> ucode loading is expected on parked CPUs, we should be able to
> determine the primary threads and the number of cores no matter it is
> online or parked. To this end, a new sibling map should be maintained
> for each CPU and this map isn't changed when a CPU gets parked.

Would this really be necessary? If any thread in a core is not
parked, bringing up the parked threads is unnecessary. If all
threads of a core are parked, simply nudge the thread with ID
zero?

> In Linux kernel, the approach is quite simple: late loading is
> prohibited if any CPU is parked; admin should online all parked CPU
> before loading ucode.

Well, this is certainly an option, but (as per above) I think
this is too rigid: Refusing the operation would be necessary
only if there's a core with all of its threads parked. (I'd
in particular like late loading to work in SMT-disabled mode.)

Jan

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