linux-kernel - Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting

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Date:   Wed, 29 Sep 2021 19:15:53 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        iommu@...ts.linux-foundation.org,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8] x86/mmu: Add mm-based PASID refcounting

On Wed, Sep 29 2021 at 09:59, Andy Lutomirski wrote:
> On 9/29/21 05:28, Thomas Gleixner wrote:
>> Looking at that patch again, none of this muck in fpu__pasid_write() is
>> required at all. The whole exception fixup is:
>> 
>>      if (!user_mode(regs))
>>               return false;
>> 
>>      if (!current->mm->pasid)
>>               return false;
>> 
>>      if (current->pasid_activated)
>>      	     return false;
>
> <-- preemption or BH here: kaboom.

Sigh, this had obviously to run in the early portion of #GP, i.e. before
enabling interrupts.

For me that's more than obvious and I apologize that I failed to mention
it.

>> 
>>      wrmsrl(MSR_IA32_PASID, current->mm->pasid);
>
> This needs the actual sane fpstate writing helper -- see other email.

And with that there is no fpstate write helper required at all.

Stop this overengineering madness already.

Thanks,

        tglx