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

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Message-ID: <8e12f79a8275ffa40858763acade4d54b3e96212.camel@xry111.site>
Date: Thu, 11 Apr 2024 13:38:33 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Andrew Cooper <andrew.cooper3@...rix.com>, Sean Christopherson
	 <seanjc@...gle.com>, Michael Kelley <mhklinux@...look.com>
Cc: Dave Hansen <dave.hansen@...el.com>, 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 Tue, 2024-04-09 at 08:56 +0100, Andrew Cooper wrote:
> 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.

So... Should I sent a v6 with the hypervisor checking reverted [ i.e.
always enable PCID if boot_cpu_has(X86_FEATURE_HYPERVISOR) ]?

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University