linux-kernel - Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO regions

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Message-ID: <20251031064851.GA74544@unreal>
Date: Fri, 31 Oct 2025 08:48:51 +0200
From: Leon Romanovsky <leon@...nel.org>
To: Alex Williamson <alex@...zbot.org>
Cc: Alex Williamson <alex.williamson@...hat.com>,
	Jason Gunthorpe <jgg@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Christian König <christian.koenig@....com>,
	dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	kvm@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linux-pci@...r.kernel.org, Logan Gunthorpe <logang@...tatee.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Vivek Kasireddy <vivek.kasireddy@...el.com>,
	Will Deacon <will@...nel.org>
Subject: Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO
 regions

On Thu, Oct 30, 2025 at 02:38:36PM -0600, Alex Williamson wrote:
> On Mon, 13 Oct 2025 18:26:11 +0300
> Leon Romanovsky <leon@...nel.org> wrote:
> > diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> > index fe247d0e2831..56b1320238a9 100644
> > --- a/drivers/vfio/pci/vfio_pci_core.c
> > +++ b/drivers/vfio/pci/vfio_pci_core.c
> > @@ -1511,6 +1520,19 @@ int vfio_pci_core_ioctl_feature(struct vfio_device *device, u32 flags,
> >  		return vfio_pci_core_pm_exit(vdev, flags, arg, argsz);
> >  	case VFIO_DEVICE_FEATURE_PCI_VF_TOKEN:
> >  		return vfio_pci_core_feature_token(vdev, flags, arg, argsz);
> > +	case VFIO_DEVICE_FEATURE_DMA_BUF:
> > +		if (device->ops->ioctl != vfio_pci_core_ioctl)
> > +			/*
> > +			 * Devices that overwrite general .ioctl() callback
> > +			 * usually do it to implement their own
> > +			 * VFIO_DEVICE_GET_REGION_INFO handlerm and they present
> 
> Typo, "handlerm"

Thanks, this part of code is going to be different in v6.

> 

<...>

> > @@ -2482,6 +2506,10 @@ static int vfio_pci_dev_set_hot_reset(struct vfio_device_set *dev_set,
> >  
> >  	ret = pci_reset_bus(pdev);
> >  
> > +	list_for_each_entry(vdev, &dev_set->device_list, vdev.dev_set_list)
> > +		if (__vfio_pci_memory_enabled(vdev))
> > +			vfio_pci_dma_buf_move(vdev, false);
> > +
> >  	vdev = list_last_entry(&dev_set->device_list,
> >  			       struct vfio_pci_core_device, vdev.dev_set_list);
> >  
> 
> This needs to be placed in the existing undo loop with the up_write(),
> otherwise it can be missed in the error case.

I'll move, but it caused me to wonder what did you want to achieve with
this "vdev = list_last_entry ..." line? vdev is overwritten immediately
after that line.

> 
> > diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > new file mode 100644
> > index 000000000000..eaba010777f3
> > --- /dev/null
> > +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> > +static unsigned int calc_sg_nents(struct vfio_pci_dma_buf *priv,
> > +				  struct dma_iova_state *state)
> > +{
> > +	struct phys_vec *phys_vec = priv->phys_vec;
> > +	unsigned int nents = 0;
> > +	u32 i;
> > +
> > +	if (!state || !dma_use_iova(state))
> > +		for (i = 0; i < priv->nr_ranges; i++)
> > +			nents += DIV_ROUND_UP(phys_vec[i].len, UINT_MAX);
> > +	else
> > +		/*
> > +		 * In IOVA case, there is only one SG entry which spans
> > +		 * for whole IOVA address space, but we need to make sure
> > +		 * that it fits sg->length, maybe we need more.
> > +		 */
> > +		nents = DIV_ROUND_UP(priv->size, UINT_MAX);
> 
> I think we're arguably running afoul of the coding style standard here
> that this is not a single simple statement and should use braces.
> 

<...>

> > +err_unmap_dma:
> > +	if (!i || !state)
> > +		; /* Do nothing */
> > +	else if (dma_use_iova(state))
> > +		dma_iova_destroy(attachment->dev, state, mapped_len, dir,
> > +				 attrs);
> > +	else
> > +		for_each_sgtable_dma_sg(sgt, sgl, i)
> > +			dma_unmap_phys(attachment->dev, sg_dma_address(sgl),
> > +					sg_dma_len(sgl), dir, attrs);
> 
> Same, here for braces.
> 

<...>

> > +	if (!state)
> > +		; /* Do nothing */
> > +	else if (dma_use_iova(state))
> > +		dma_iova_destroy(attachment->dev, state, priv->size, dir,
> > +				 attrs);
> > +	else
> > +		for_each_sgtable_dma_sg(sgt, sgl, i)
> > +			dma_unmap_phys(attachment->dev, sg_dma_address(sgl),
> > +				       sg_dma_len(sgl), dir, attrs);
> > +
> 
> Here too.

I will change it, but it is worth to admit that I'm consistent in my
coding style.

> 
> > +	sg_free_table(sgt);
> > +	kfree(sgt);
> > +}
> ...
> > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > index 75100bf009ba..63214467c875 100644
> > --- a/include/uapi/linux/vfio.h
> > +++ b/include/uapi/linux/vfio.h
> > @@ -1478,6 +1478,31 @@ struct vfio_device_feature_bus_master {
> >  };
> >  #define VFIO_DEVICE_FEATURE_BUS_MASTER 10
> >  
> > +/**
> > + * Upon VFIO_DEVICE_FEATURE_GET create a dma_buf fd for the
> > + * regions selected.
> > + *
> > + * open_flags are the typical flags passed to open(2), eg O_RDWR, O_CLOEXEC,
> > + * etc. offset/length specify a slice of the region to create the dmabuf from.
> > + * nr_ranges is the total number of (P2P DMA) ranges that comprise the dmabuf.
> > + *
> 
> Probably worth noting that .flags should be zero, I see we enforce
> that.  Thanks,

Added, thanks

> 
> Alex
>