linux-kernel - Re: [PATCH] x86/cpufeatures: Force disable X86_FEATURE_ENQCMD and remove update

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Date:   Thu, 3 Jun 2021 10:31:36 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Luck, Tony" <tony.luck@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Cc:     "Yu, Fenghua" <fenghua.yu@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Mehta, Sohil" <sohil.mehta@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH] x86/cpufeatures: Force disable X86_FEATURE_ENQCMD and
 remove update_pasid()

On 6/2/21 1:37 PM, Luck, Tony wrote:
>>> ... so on a PASID system, your trivial reproducer would theoretically
>>> fire the same way and corrupt FPU state just as well.
>>
>> This is worse and you can't selftest it because the IPI can just hit in
>> the middle of _any_ FPU state operation and corrupt state.
> 
> That sounds like we should abandon the "IPI all the other threads
> to force enable the PASID for them" approach. It would just be a
> nightmare of papering over cracks when the IPI was delivered at
> some inconvenient moment when the recipient was in the middle
> of touching xsave state.
> 
> I've told Fenghua to dig out the previous iteration of this patch where
> the plan was to lazily fix the PASID_MSR in other threads in the #GP
> handler.

Blech.  Also this won't work for other PASID-like features.

I have a half-written patch to fix this up for real.  Stay tuned.

> Seems like a better direction than trying to fix the IPI method. The virtualization
> folks will like this way more because IPI in guest causes a couple of VMEXIT
> so is somewhat expensive.

It happens at most once per PASID-using process.