linux-kernel - Re: [PATCH RFCv2 03/13] iommu: Make iommu_dma_prepare

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Message-ID: <20250123181657.GT5556@nvidia.com>
Date: Thu, 23 Jan 2025 14:16:57 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Eric Auger <eric.auger@...hat.com>
Cc: Nicolin Chen <nicolinc@...dia.com>, will@...nel.org,
	robin.murphy@....com, kevin.tian@...el.com, tglx@...utronix.de,
	maz@...nel.org, alex.williamson@...hat.com, joro@...tes.org,
	shuah@...nel.org, reinette.chatre@...el.com, yebin10@...wei.com,
	apatel@...tanamicro.com, shivamurthy.shastri@...utronix.de,
	bhelgaas@...gle.com, anna-maria@...utronix.de, yury.norov@...il.com,
	nipun.gupta@....com, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
	patches@...ts.linux.dev, jean-philippe@...aro.org, mdf@...nel.org,
	mshavit@...gle.com, shameerali.kolothum.thodi@...wei.com,
	smostafa@...gle.com, ddutile@...hat.com
Subject: Re: [PATCH RFCv2 03/13] iommu: Make iommu_dma_prepare_msi() into a
 generic operation

On Thu, Jan 23, 2025 at 06:10:47PM +0100, Eric Auger wrote:
> Hi,
> 
> 
> On 1/11/25 4:32 AM, Nicolin Chen wrote:
> > From: Jason Gunthorpe <jgg@...dia.com>
> >
> > SW_MSI supports IOMMU to translate an MSI message before the MSI message
> > is delivered to the interrupt controller. On such systems the iommu_domain
> > must have a translation for the MSI message for interrupts to work.
> >
> > The IRQ subsystem will call into IOMMU to request that a physical page be
> > setup to receive MSI message, and the IOMMU then sets an IOVA that maps to
> > that physical page. Ultimately the IOVA is programmed into the device via
> > the msi_msg.
> >
> > Generalize this to allow the iommu_domain owner to provide its own
> > implementation of this mapping. Add a function pointer to struct
> > iommu_domain to allow the domain owner to provide an implementation.
> >
> > Have dma-iommu supply its implementation for IOMMU_DOMAIN_DMA types during
> > the iommu_get_dma_cookie() path. For IOMMU_DOMAIN_UNMANAGED types used by
> > VFIO (and iommufd for now), have the same iommu_dma_sw_msi set as well in
> > the iommu_get_msi_cookie() path.
> >
> > Hold the group mutex while in iommu_dma_prepare_msi() to ensure the domain
> > doesn't change or become freed while running. Races with IRQ operations
> > from VFIO and domain changes from iommufd are possible here.
> this was my question in previous comments

Ah, well there is the answer :)

> > Rreplace the msi_prepare_lock with a lockdep assertion for the group mutex
> Replace
> > as documentation. For the dma_iommu.c each iommu_domain unique to a
> is?
> > group.

Yes

Replace the msi_prepare_lock with a lockdep assertion for the group mutex
as documentation. For the dmau_iommu.c each iommu_domain is unique to a
group.

> > @@ -443,6 +449,9 @@ void iommu_put_dma_cookie(struct iommu_domain *domain)
> >  	struct iommu_dma_cookie *cookie = domain->iova_cookie;
> >  	struct iommu_dma_msi_page *msi, *tmp;
> >  
> > +	if (domain->sw_msi != iommu_dma_sw_msi)
> > +		return;
> > +
> I don't get the above check.

It is because of this:

void iommu_domain_free(struct iommu_domain *domain)
{
	if (domain->type == IOMMU_DOMAIN_SVA)
		mmdrop(domain->mm);
	iommu_put_dma_cookie(domain);

iommufd may be using domain->sw_msi so iommu_put_dma_cookie() needs to
be a NOP. Also, later we move cookie into a union so it is not
reliably NULL anymore.

> The comment says this is also called for a
> cookie prepared with iommu_get_dma_cookie(). Don't you need to do some
> cleanup for this latter?

That seems seems OK, only two places set domain->iova_cookie:

int iommu_get_dma_cookie(struct iommu_domain *domain)
{
	domain->iova_cookie = cookie_alloc(IOMMU_DMA_IOVA_COOKIE);
	iommu_domain_set_sw_msi(domain, iommu_dma_sw_msi);

and

int iommu_get_msi_cookie(struct iommu_domain *domain, dma_addr_t base)
{
	domain->iova_cookie = cookie;
	iommu_domain_set_sw_msi(domain, iommu_dma_sw_msi);

So (domain->sw_msi == iommu_dma_sw_msi) in iommu_put_dma_cookie() for
both cases..

Jason