linux-kernel - Re: [PATCH v2 1/1] iommu/sva: Invalidate KVA range on kernel TLB flush

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Message-ID: <20250716125725.37aa3f38@pumpkin>
Date: Wed, 16 Jul 2025 12:57:25 +0100
From: David Laight <david.laight.linux@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Lu Baolu <baolu.lu@...ux.intel.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>, Kevin
 Tian <kevin.tian@...el.com>, Jason Gunthorpe <jgg@...dia.com>, Jann Horn
 <jannh@...gle.com>, Vasant Hegde <vasant.hegde@....com>, Dave Hansen
 <dave.hansen@...el.com>, Alistair Popple <apopple@...dia.com>, Uladzislau
 Rezki <urezki@...il.com>, Jean-Philippe Brucker <jean-philippe@...aro.org>,
 Andy Lutomirski <luto@...nel.org>, "Tested-by : Yi Lai"
 <yi1.lai@...el.com>, iommu@...ts.linux.dev, security@...nel.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/1] iommu/sva: Invalidate KVA range on kernel TLB
 flush

On Thu, 10 Jul 2025 17:53:19 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Jul 10, 2025 at 03:54:32PM +0200, Peter Zijlstra wrote:
> 
> > > @@ -132,8 +136,15 @@ struct iommu_sva *iommu_sva_bind_device(struct device *dev, struct mm_struct *mm
> > >  	if (ret)
> > >  		goto out_free_domain;
> > >  	domain->users = 1;
> > > -	list_add(&domain->next, &mm->iommu_mm->sva_domains);
> > >  
> > > +	if (list_empty(&iommu_mm->sva_domains)) {
> > > +		scoped_guard(spinlock_irqsave, &iommu_mms_lock) {
> > > +			if (list_empty(&iommu_sva_mms))
> > > +				static_branch_enable(&iommu_sva_present);
> > > +			list_add(&iommu_mm->mm_list_elm, &iommu_sva_mms);
> > > +		}
> > > +	}
> > > +	list_add(&domain->next, &iommu_mm->sva_domains);
> > >  out:
> > >  	refcount_set(&handle->users, 1);
> > >  	mutex_unlock(&iommu_sva_lock);
> > > @@ -175,6 +186,15 @@ void iommu_sva_unbind_device(struct iommu_sva *handle)
> > >  		list_del(&domain->next);
> > >  		iommu_domain_free(domain);
> > >  	}
> > > +
> > > +	if (list_empty(&iommu_mm->sva_domains)) {
> > > +		scoped_guard(spinlock_irqsave, &iommu_mms_lock) {
> > > +			list_del(&iommu_mm->mm_list_elm);
> > > +			if (list_empty(&iommu_sva_mms))
> > > +				static_branch_disable(&iommu_sva_present);
> > > +		}
> > > +	}
> > > +
> > >  	mutex_unlock(&iommu_sva_lock);
> > >  	kfree(handle);
> > >  }  
> > 
> > This seems an odd coding style choice; why the extra unneeded
> > indentation? That is, what's wrong with:
> > 
> > 	if (list_empty()) {
> > 		guard(spinlock_irqsave)(&iommu_mms_lock);
> > 		list_del();
> > 		if (list_empty()
> > 			static_branch_disable();
> > 	}  
> 
> Well, for one, you can't do static_branch_{en,dis}able() from atomic
> context...

Aren't they also somewhat expensive - so you really want to use them
for configuration options which pretty much don't change.

	David

> 
> Was this ever tested?
>