linux-kernel - Re: [PATCH v4 12/12] x86/traps: Fix up invalid PASID

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Date:   Fri, 26 Jun 2020 11:16:24 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>, H Peter Anvin <hpa@...or.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Christoph Hellwig <hch@...radeed.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Jun Pan <jacob.jun.pan@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Sohil Mehta <sohil.mehta@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v4 12/12] x86/traps: Fix up invalid PASID

Hi, Peter,

On Fri, Jun 26, 2020 at 11:44:50AM +0200, Peter Zijlstra wrote:
> On Thu, Jun 25, 2020 at 01:17:22PM -0700, Fenghua Yu wrote:
> 
> > +static bool fixup_pasid_exception(void)
> > +{
> > +	if (!IS_ENABLED(CONFIG_INTEL_IOMMU_SVM))
> > +		return false;
> > +	if (!static_cpu_has(X86_FEATURE_ENQCMD))
> > +		return false;
> 
> elsewhere you had another variation:
> 
> +       if (!IS_ENABLED(CONFIG_INTEL_IOMMU_SVM))
> +               return;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_ENQCMD))
> +               return;
> 
> Which is it, and why do we need the CONFIG thing when combined with the
> enabled thing?
> 

I will use the second one with cpu_feature_enabled() for both cases.

The CONFIG thing is for compilation time optimization when
CONFIG_INTEL_IOMMU_SVM is not set.

If CONFIG_INTEL_IOMMU_SVM is not set, IS_ENABLED(CONFIG_INTEL_IOMMU_SVM) 
is "false" during compilation time. Then GCC will optimize 
fixup_pasid_execption() to empty and will not define
__fixup_pasid_exception() at all because no one calls it.

If CONFIG_INTEL_IOMMU_SVM is set, IS_ENABLED(...) is always true.
Depending on cpu_feature_enabled(X86_FEATURE_ENQCMD), __fixup_pasid_execption()
will be called or not during run time.

Does it make sense?

Do you want me to define a helper enqcmd_enabled()?

static inline bool enqcmd_enabled(void)
{
   if (!IS_ENABLED(CONFIG_INTEL_IOMMU_SVM))
           return false;
   if (!static_cpu_has(X86_FEATURE_ENQCMD))
           return false;
   return true;
}

Then both fixup_pasid_execption() and free_pasid() can call it.

static bool fixup_pasid_exception(void)
{
	if (!enqcmd_enabled())
		return false;
	
	return __fixup_pasid_exception();
}

statis inline void free_pasid(struct m_struct *mm)
{
	if (!enqcmd_enabled())
		return;

	__free_pasid(mm);
}

Please advice.

-Fenghua