linux-kernel - Re: [PATCH v2 7/7] iommu: Turn iova_cookie to dma-iommu private pointer

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Message-ID: <Z79j/DblkTGNRDg+@Asurada-Nvidia>
Date: Wed, 26 Feb 2025 10:57:00 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <kevin.tian@...el.com>, <tglx@...utronix.de>, <maz@...nel.org>,
	<joro@...tes.org>, <will@...nel.org>, <robin.murphy@....com>,
	<shuah@...nel.org>, <iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kselftest@...r.kernel.org>,
	<eric.auger@...hat.com>, <baolu.lu@...ux.intel.com>, <yi.l.liu@...el.com>,
	<yury.norov@...il.com>, <jacob.pan@...ux.microsoft.com>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v2 7/7] iommu: Turn iova_cookie to dma-iommu private
 pointer

On Wed, Feb 26, 2025 at 01:36:10PM -0400, Jason Gunthorpe wrote:
> On Tue, Feb 25, 2025 at 06:25:27PM -0800, Nicolin Chen wrote:
> > On Fri, Feb 21, 2025 at 10:39:59AM -0400, Jason Gunthorpe wrote:
> > > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > > index 99dd72998cb7f7..082274e8ba6a3d 100644
> > > --- a/include/linux/iommu.h
> > > +++ b/include/linux/iommu.h
> > > @@ -1534,12 +1534,16 @@ void iommu_debugfs_setup(void);
> > >  static inline void iommu_debugfs_setup(void) {}
> > >  #endif
> > >  
> > > -#ifdef CONFIG_IOMMU_DMA
> > > +#if defined(CONFIG_IOMMU_DMA) && IS_ENABLED(CONFIG_IRQ_MSI_IOMMU)
> > >  int iommu_get_msi_cookie(struct iommu_domain *domain, dma_addr_t base);
> > > +void iommu_put_msi_cookie(struct iommu_domain *domain);
> > >  #else /* CONFIG_IOMMU_DMA */
> > >  static inline int iommu_get_msi_cookie(struct iommu_domain *domain, dma_addr_t base)
> > >  {
> > > -	return -ENODEV;
> > > +	return 0;
> > 
> > Should we keep the -ENODEV here for !CONFIG_IOMMU_DMA?
> 
> My feeling was if the system doesn't have an IRQ driver that needs
> MSI_IOMMU but does have a IOMMU driver that reports SW_MSI reserved
> regions then iommufd/vfio should not fail.

OK, I see. But we are also changing the behavior for the
!CONFIG_IOMMU_DMA configuration, in which case all other iommu
functions seem to return -ENODEV. And I assume we would need a
justification for such a change?

Perhaps, this can be explicit, just to keep the consistency:
	/* NOP if IOMMU driver reports SW_MSI reserved regions */
	return IS_ENABLED(CONFIG_IOMMU_DMA) ? 0 : -ENODEV;

> I don't think it is realistic that we'd ever hit this return.

Yea, the only caller is VFIO, where there are quite a few IOMMU
functions calls before reaching to this one. So, it would have
been just returned with any -ENODEV prior.

Thanks
Nicolin