linux-kernel - RE: [PATCH RFC 0/3] MAP_POPULATE for device memory

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Message-ID: <7f46ef3c80734f478501d21cef0182c5@AcuMS.aculab.com>
Date:   Sun, 6 Mar 2022 08:30:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jarkko Sakkinen' <jarkko@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
CC:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Nathaniel McCallum <nathaniel@...fian.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Matthew Auld <matthew.auld@...el.com>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jason Ekstrand <jason@...kstrand.net>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
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Subject: RE: [PATCH RFC 0/3] MAP_POPULATE for device memory

From: Jarkko Sakkinen
> Sent: 06 March 2022 05:32
> 
> For device memory (aka VM_IO | VM_PFNMAP) MAP_POPULATE does nothing. Allow
> to use that for initializing the device memory by providing a new callback
> f_ops->populate() for the purpose.
> 
> SGX patches are provided to show the callback in context.
> 
> An obvious alternative is a ioctl but it is less elegant and requires
> two syscalls (mmap + ioctl) per memory range, instead of just one
> (mmap).

Is this all about trying to stop the vm_operations_struct.fault()
function being called?

It is pretty easy to ensure the mappings are setup in the driver's
mmap() function.
Then the fault() function can just return -VM_FAULT_SIGBUS;

If it is actually device memory you just need to call vm_iomap_memory()
That quite nicely mmap()s PCIe memory space into a user process.

Mapping driver memory is slightly more difficult.
For buffers allocated with dma_alloc_coherent() you can
probably use dma_mmap_coherent().
But I have a loop calling remap_pfn_range() because the
buffer area is made of multiple 16kB kernel buffers that
need to be mapped to contiguous user pages.

	David

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