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

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Message-ID: <YVSdZGhbiqXbUaXV@otcwcpicx3.sc.intel.com>
Date:   Wed, 29 Sep 2021 17:07:48 +0000
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "iommu@...ts.linux-foundation.org" <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

Hi, Thomas,

On Wed, Sep 29, 2021 at 09:51:15AM -0700, Luck, Tony wrote:
> > There is zero requirement to look at TIF_NEED_FPU_LOAD or
> > fpregs_state_valid() simply because the #GP comes straight from user
> > space which means the FPU registers contain the current tasks user space
> > state.
> 
> Just to double confirm ... there is no point in the #GP handler up to this point
> where pre-emption can occur?

Same question here. The fixup function is called after cond_local_irq_enable().
If an interrupt comes before fixup_pasid_exception(), the interrupt may
use FPU and call kernel_fpu_begin_mask()->set(TIF_NEED_FPU_LOAD)->
__cpu_invalidate_fpregs_state(). Then writing to the IA32_PASID MSR. When
exiting to user, the FPU states will be restored to the FPU regs including
the IA32_PASID MSR. So the MSR could be different from the value written in
fixup_pasid_execption(). Is it possible?

Or should fixup_pasid_exception() be called before cond_local_irq_enable()?

Thanks.

-Fenghua