linux-kernel - Re: [PATCH v8 2/2] x86/mm: Don't disable PCID if the kernel is running on a hypervisor

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Date: Wed, 17 Apr 2024 10:25:33 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Sean Christopherson <seanjc@...gle.com>
Cc: Xi Ruoyao <xry111@...111.site>, Dave Hansen
 <dave.hansen@...ux.intel.com>, Michael Kelley <mhklinux@...look.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, linux-kernel@...r.kernel.org,
 Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v8 2/2] x86/mm: Don't disable PCID if the kernel is
 running on a hypervisor

On 4/17/24 10:22, Pawan Gupta wrote:
>>>  static const struct x86_cpu_id invlpg_miss_ids[] = {
>>> +	/* Only bare-metal is affected.  PCIDs in guests are OK.  */
>>> +	{
>>> +	  .vendor	= X86_VENDOR_INTEL,
>>> +	  .family	= 6,
>>> +	  .model	= INTEL_FAM6_ANY,
>>> +	  .feature	= X86_FEATURE_HYPERVISOR,
>> Isn't this inverted?  x86_match_cpu() will return NULL if the CPU doesn't have
>> HYPERVISOR.  We want it to return NULL if the CPU *does* have HYPERVISOR.
> I think the implementation is correct, x86_match_cpu() will not return
> NULL if the CPU doesn't have HYPERVISOR feature *and* matches one of the
> CPUs below. It will only return NULL if none of the entries match.

I think I gave a crappy suggestion here.

Let's just do the X86_FEATURE_HYPERVISOR explicitly in the code instead
of trying to cram it into the invlpg_miss_ids[] check.  It's way easier
to understand with an explicit code check.