linux-kernel - Re: [PATCH v2] x86/mm: Don't disable INVLPG if "incomplete Global INVLPG flushes" is fixed by microcode

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Date: Tue, 9 Apr 2024 08:56:28 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Michael Kelley <mhklinux@...look.com>
Cc: Dave Hansen <dave.hansen@...el.com>, Xi Ruoyao <xry111@...111.site>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
 "x86@...nel.org" <x86@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Dexuan Cui <decui@...rosoft.com>
Subject: Re: [PATCH v2] x86/mm: Don't disable INVLPG if "incomplete Global
 INVLPG flushes" is fixed by microcode

On 09/04/2024 2:43 am, Sean Christopherson wrote:
> On Mon, Apr 08, 2024, Michael Kelley wrote:
>> From: Dave Hansen <dave.hansen@...el.com> Sent: Thursday, April 4, 2024 11:09 AM
>>> On 4/4/24 10:48, Michael Kelley wrote:
>>>> I agree one could argue that it is a hypervisor bug to present PCID to the guest
>>>> in this situation. It's a lot cleaner to not have a guest be checking FMS and
>>>> microcode versions. But whether that's practical in the real world, at least
>>>> for Hyper-V, I don't know. What's the real impact of running with PCID while
>>>> the flaw is still present? I don’t know the history here ...
>>> There's a chance that INVLPG will appear ineffective.
>>>
>>> The bad sequence would go something like this: The kernel does the
>>> INVLPG on a global mapping.  Later, when switching PCIDs, the TLB entry
>>> mysteriously reappears.  No PCIDs switching means no mysterious
>>> reappearance.
>> Xi Ruoyao's patch identifies these errata:  RPL042 and ADL063.  In the links
>> to the documents Xi provided, both of these errata have the following
>> statement in the Errata Details section:
>>
>>     This erratum does not apply in VMX non-root operation.  It applies only
>>     when PCIDs are enabled and either in VMX root operation or outside
>>     VMX operation.
>>
>> I don't have deep expertise on the terminology here, but this sounds
>> like it is saying the erratum doesn’t apply in a guest VM.  Or am I
>> misunderstanding?
> Huh.  My read of that is the same as yours.  If that's the case, then it probably
> makes sense to have KVM advertise support if PCID is available in hardware, even
> if PCID is disabled by the host kernel.

My reading is the same also.  Seems like VMs are fine.

~Andrew