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: Mon, 8 Apr 2024 23:31:25 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Dave Hansen <dave.hansen@...el.com>, Sean Christopherson
	<seanjc@...gle.com>, Andrew Cooper <andrew.cooper3@...rix.com>
CC: 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

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?

Michael