linux-kernel - Re: [PATCH v2 06/12] iommu: Make dev->fault

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Message-ID: <ZNUwgjJ+2GHf2MOW@ziepe.ca>
Date:   Thu, 10 Aug 2023 15:46:26 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Tian <kevin.tian@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        Yi Liu <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux.dev, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 06/12] iommu: Make dev->fault_param static

On Thu, Aug 10, 2023 at 03:20:07PM -0300, Jason Gunthorpe wrote:
> On Thu, Jul 27, 2023 at 01:48:31PM +0800, Lu Baolu wrote:
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 4ba3bb692993..3e4ff984aa85 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -302,7 +302,15 @@ static int dev_iommu_get(struct device *dev)
> >  		return -ENOMEM;
> >  
> >  	mutex_init(&param->lock);
> > +	param->fault_param = kzalloc(sizeof(*param->fault_param), GFP_KERNEL);
> > +	if (!param->fault_param) {
> > +		kfree(param);
> > +		return -ENOMEM;
> > +	}
> > +	mutex_init(&param->fault_param->lock);
> > +	INIT_LIST_HEAD(&param->fault_param->faults);
> >  	dev->iommu = param;
> 
> This allocation seems pointless?
> 
> If we always allocate the fault param then just don't make it a
> pointer in the first place.
> 
> The appeal of allocation would be to save a few bytes in the common
> case that the driver doesn't support faulting.
> 
> Which means the driver needs to make some call to enable faulting for
> a device. In this case I'd continue to lazy free on release like this
> patch does.

For instance allocate the fault_param in iopf_queue_add_device() which
is the only thing that needs it.

Actually probably just merge struct iopf_device_param and
iommu_fault_param ?

When you call iopf_queue_add_device() it enables page faulting mode,
does 1 additional allocation for all additional required per-device
memory and thats it.

Jason