linux-kernel - Re: [PATCH v9 6/7] iommu/arm-smmu-v3: Add arm_smmu_invs based arm_smmu_domain_inv

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Message-ID: <aXj+zaLoh/yQmHOH@Asurada-Nvidia>
Date: Tue, 27 Jan 2026 10:07:09 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Pranjal Shrivastava <praan@...gle.com>, <will@...nel.org>,
	<jean-philippe@...aro.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<balbirs@...dia.com>, <miko.lenczewski@....com>, <peterz@...radead.org>,
	<kevin.tian@...el.com>, <linux-arm-kernel@...ts.infradead.org>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 6/7] iommu/arm-smmu-v3: Add arm_smmu_invs based
 arm_smmu_domain_inv_range()

On Tue, Jan 27, 2026 at 01:08:37PM -0400, Jason Gunthorpe wrote:
> On Tue, Jan 27, 2026 at 08:38:31AM -0800, Nicolin Chen wrote:
> > Hi Pranjal,
> > 
> > Sorry, I missed this!
> > 
> > On Fri, Jan 23, 2026 at 09:48:37AM +0000, Pranjal Shrivastava wrote:
> > > On Fri, Dec 19, 2025 at 12:11:28PM -0800, Nicolin Chen wrote:
> > > > +	/*
> > > > +	 * Avoid locking unless ATS is being used. No ATC invalidation can be
> > > > +	 * going on after a domain is detached.
> > > > +	 */
> > > > +	if (invs->has_ats) {
> > > > +		read_lock(&invs->rwlock);
> > > 
> > > Shouldn't these be read_lock_irqsave for all rwlock variants here? 
> > > Invalidations might happen in IRQ context as well..
> > > 
> > > > +		__arm_smmu_domain_inv_range(invs, iova, size, granule, leaf);
> > > > +		read_unlock(&invs->rwlock);
> > 
> > It was kept from the older versions where we had a trylock. Jason
> > had an insight about this, mainly for less latency on invalidation
> > threads.
> > 
> > Yet, now we have a plain locking. TBH, I can't find a good reason
> > justifying this. And it does look a bit unsafe to me. So, I think
> > I will just change to the _irqsave version. (Jason?)
> 
> My understanding has been that this invalidation can run from an IRQ
> context - we permit the use of the DMA API from an interrupt handler?
> 
> I though that for rwsem the read side does not require the _irqsave,
> even if it is in an irq context, unless the write side runs from an
> IRQ. 

Hmm, is "rwsem" a typo? Because it's rwlock_t, which is spinlock :-/

> Here the write side always runs from a process context.
> 
> So the write side will block the IRQ which ensures we don't spin
> during read in an IRQ.

And, does write_lock_irqsave() disable global IRQ or local IRQ only?

Documentation/locking/locktypes.rst mentions "local_irq_disable()"..

Thanks
Nicolin