Re: [PATCH v2] x86/ucode: Refresh raw CPU policy after microcode load

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
	Jan Beulich <jbeulich@suse.com>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH v2] x86/ucode: Refresh raw CPU policy after microcode load
Date: Thu, 4 May 2023 12:00:07 +0100	[thread overview]
Message-ID: <e9e60c7a-2db5-3985-356e-3a039e44d1a1@citrix.com> (raw)
In-Reply-To: <ZFONUdkG2ow9uckX@Air-de-Roger>

On 04/05/2023 11:47 am, Roger Pau Monné wrote:
> On Thu, May 04, 2023 at 11:26:07AM +0100, Andrew Cooper wrote:
>> Loading microcode can cause new features to appear.  This has happened
>> routinely since Spectre/Meltdown, and even the presence of new status bits can
>> sometimes mean the administrator has no further actions to perform.
>>
>> Conversely, loading microcode can occasionally cause features to disappear.
>> As with livepatching, it is very much the administrators responsibility to
>> confirm that a late microcode load is safe on the intended system before
>> rolling it out in production.
>>
>> Refresh the raw CPU policy after late microcode load appears to have done
>> something, so xen-cpuid can reflect the updated state of the system.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> Acked-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks.

> I'm not fully sure it's worth calling calculate_raw_cpu_policy() if
> updated != nr_cores, as it's possible the current CPU where the policy
> is regenerated hasn't been updated.

As said previously, updated != nr_cores may exist in principle, but it
is not a corner case we should care about.

If the system really is asymmetric after a late load, then it is
probably not long for the world.  There is nothing Xen can do to recover.

It is awkward that we're not on the BSP when collecting.  I think that
part of the logic didn't survive the rebase over switching to
stop-machine.  I'll add it to the gitlab ticket of minor ucode work,
because it's not the end of the world.

~Andrew