linux-kernel - Re: [PATCH 1/1] arch/x86/microcode/intel: Remove unnecessary cache writeback and invalidation

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Message-ID: <ZoWZG75hFpfK6kkv@a4bf019067fa.jf.intel.com>
Date: Wed, 3 Jul 2024 13:50:37 -0700
From: Ashok Raj <ashok.raj@...el.com>
To: Dave Hansen <dave.hansen@...el.com>
CC: "Chang S. Bae" <chang.seok.bae@...el.com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <tony.luck@...el.com>, Yan Hua Wu
	<yanhua1.wu@...el.com>, William Xie <william.xie@...el.com>, Ashok Raj
	<ashok.raj@...el.com>
Subject: Re: [PATCH 1/1] arch/x86/microcode/intel: Remove unnecessary cache
 writeback and invalidation

On Mon, Jul 01, 2024 at 03:56:20PM -0700, Dave Hansen wrote:
> On 7/1/24 14:20, Chang S. Bae wrote:
> ...> Remove native_wbinvd() and update the erratum name to align with the
> > latest errata documentation.
> 
> I'm all for simplifying this code and also for removing any WBINVDs that
> we possibly can.  But it also makes me a wee bit nervous that this could
> have been hiding any _new_ issues (like the Broadwell one) had they
> crept in.
> 
> I'm tentatively in favor of this, but it's definitely the kind of thing
> we want to apply early and get maximum testing on.
> 
> I'd also appreciate an ack from Ashok on this one.

Thanks Dave. At the time when wbinvd() was added, there was no
confirmation that we didn't have any other products that slipped. This was
also during the meltdown timeframe, so big hammer was choosen for safety.

We attempted to remove them during the minrev rework.

https://lore.kernel.org/lkml/20230130213955.6046-9-ashok.raj@intel.com/

Agree that we must get wider testing. Only caveat is that you should find a
newer microcode to apply, which might be difficult for all products. Unless
there is a debug option to reload force the same rev in case you don't have
a newer ucode to test. Its good to get this in to reduce the big hammer
effect.

Acked by: Ashok Raj <ashok.raj@...el.com>

-- 
Cheers,
Ashok