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

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Date:   Wed, 29 Sep 2021 18:07:22 +0000
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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

Hi, Tony,

On Wed, Sep 29, 2021 at 10:41:42AM -0700, Luck, Tony wrote:
> On Wed, Sep 29, 2021 at 07:15:53PM +0200, Thomas Gleixner wrote:
> > 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.
> 
> Like this? Obviously with some comment about why this is being done.
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index a58800973aed..a848a59291e7 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -536,6 +536,12 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
>  	unsigned long gp_addr;
>  	int ret;
>  

Add
+#ifdef CONFIG_IOMMU_SUPPORT
because mm->pasid and current-pasid_activated are defined only if
CONFIG_IOMMU_SUPPORT is defined.

> +	if (user_mode(regs) && current->mm->pasid && !current->pasid_activated) {

Maybe need to add "&& cpu_feature_enabled(X86_FEATURE_ENQCMD)" because
the IA32_PASID MSR is only used when ENQCMD is enabled?

> +		current->pasid_activated = 1;
> +		wrmsrl(MSR_IA32_PASID, current->mm->pasid | MSR_IA32_PASID_VALID);
> +		return;
> +	}
> +

+endif

>  	cond_local_irq_enable(regs);
>  
>  	if (static_cpu_has(X86_FEATURE_UMIP)) {

Thanks.

-Fenghua