linux-kernel - Re: [PATCH] perf/x86: check ucode before disabling PEBS on SandyBridge

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Message-ID: <4FD7A99F.8030007@zytor.com>
Date:	Tue, 12 Jun 2012 13:42:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...64.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>,
	linux-kernel@...r.kernel.org, andi@...stfloor.org, mingo@...e.hu,
	ming.m.lin@...el.com, "Yu, Fenghua" <fenghua.yu@...el.com>
Subject: Re: [PATCH] perf/x86: check ucode before disabling PEBS on SandyBridge

On 06/12/2012 01:37 PM, Borislav Petkov wrote:
> On Tue, Jun 12, 2012 at 01:28:55PM -0700, H. Peter Anvin wrote:
>> On 06/12/2012 12:49 PM, Borislav Petkov wrote:
>>>
>>> Also, I'd assume we need some of the current functionality to be able to
>>> load ucode without rebooting the box.
>>>
>>
>> Yes, the basic idea is that we'll keep the ucode around anyway so we can
>> feed it to hotplug CPUs or S3 resume.  If the ucode is updated from
>> userspace we'd replace the in-memory copy and load it into the CPUs.
> 
> You say CPUs, i.e. plural. What is your take on loading ucode on a
> single core, i.e. the /sys/devices/system/cpu/cpuX/microcode/reload
> interface? Do you guys need to load ucode only system-wide or can you
> stomach per-core loads?
> 
> I want to disable it (or even remove it, modulo the whole userspace
> compat blah blah) on AMD.
> 
> Also, if we do system-wide, can we have a single sysfs node where we
> give the new ucode from userspace instead of per-cpu files?
> 

I personally don't know of any valid use case for per-core loads, and it
sounds like a horrid idea.  And yes, I would prefer a single sysfs file,
or better yet a plain old device node.

	-hpa

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