linux-kernel - Re: [PATCH 4/8] x86/traps: Demand-populate PASID MSR via #GP

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Message-ID: <YVO+WAqroakEYI/D@otcwcpicx3.sc.intel.com>
Date:   Wed, 29 Sep 2021 01:16:08 +0000
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     "Hansen, Dave" <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        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 4/8] x86/traps: Demand-populate PASID MSR via #GP

Hi, Tony,

On Tue, Sep 28, 2021 at 06:06:52PM -0700, Luck, Tony wrote:
> >> 	fpregs_lock();
> >
> > I'm afraid we may hit the same locking issue when we send IPI to notify another task to modify its
> > PASID state. Here the API is called to modify another running task's PASID state as well without a right lock.
> > fpregs_lock() is not enough to deal with this, I'm afraid.
> 
> We don't send IPI any more to change PASID state. The only place that the
> current patch series touches the PASID MSR is in the #GP fault handler.

It's safe for the helpers to handle the PASID case (modifying the current task's
PASID state in #GP).

But the helpers seem to be generic. They take "task" as a parameter and
handle the task as non-current case. So the helpers are not for PASID only.
They may be used by others to modify a running task's FPU state. But
It's not safe to do so.

At least need some comments/restriction for the helpers to be used on
a running task?

Thanks.

-Fenghua