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: Thu, 4 Apr 2024 11:08:55 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Michael Kelley <mhklinux@...look.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

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.