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Date:   Mon, 15 Jun 2020 11:55:29 -0700
From:   Fenghua Yu <fenghua.yu@intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Frederic Barrat <fbarrat@linux.ibm.com>,
        Andrew Donnellan <ajd@linux.ibm.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Jacob Jun Pan <jacob.jun.pan@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Sohil Mehta <sohil.mehta@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, iommu@lists.linux-foundation.org,
        amd-gfx <amd-gfx@lists.freedesktop.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v2 12/12] x86/traps: Fix up invalid PASID
Message-ID: <20200615185529.GD13792@romley-ivt3.sc.intel.com>
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Hi, Peter,

On Mon, Jun 15, 2020 at 08:31:16PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 15, 2020 at 11:12:59AM -0700, Fenghua Yu wrote:
> > > I don't get why you need a rdmsr here, or why not having one would
> > > require a TIF flag. Is that because this MSR is XSAVE/XRSTOR managed?
> > 
> > My concern is TIF flags are precious (only 3 slots available). Defining
> > a new TIF flag may be not worth it while rdmsr() can check if PASID
> > is valid in the MSR. And performance here might not be a big issue
> > in #GP.
> > 
> > But if you think using TIF flag is better, I can define a new TIF flag
> > and maintain it per thread (init 0 when clone()/fork(), set 1 in fixup()).
> > Then we can avoid using rdmsr() to check valid PASID in the MSR.
> 
> WHY ?!?! What do you need a TIF flag for?

We need "a way" to check if the per thread MSR has a valid PASID. If yes,
no need to fix up the MSR (wrmsr()), and let other handler to handle the #GP.
Otherwise, apply the heuristics and fix up the MSR and exit the #GP.

The way to check the valid PASID in the MSR is rdmsr() in this series.
A TIF flag will be much faster than rdmsr() and seems a sutiable way
to check valid PASID status per thread. That's why it could replace
rdmsr() to check PASID in the MSR.

Or do you suggest to add a random new flag in struct thread_info instead
of a TIF flag?

Thanks.

-Fenghua