Mapping IO memory read-write

Mapping IO memory read-write

[Jethro Beekman]

I'm writing a device driver for a memory-mapped device on x86-64. I'm mapping
the device in the kernel using ioremap_cache(). My file_operations.mmap function
is as follows:

static int dev_mmap(struct file *filep, struct vm_area_struct *vma) {
	vma->vm_page_prot.pgprot|=_PAGE_BIT_RW;
	return vm_iomap_memory(vma,START,LEN);
}

The user process calls mmap(..,PROT_READ|PROT_WRITE|PROT_EXEC,..). The mapping
in /proc/[pid]/maps shows the write bit. However, when looking at the actual
page table entry does not have the RW bit set. For example:

virtual address = 0000_0001_8000_0000
cr3 = 0000_0000_700a_e000
phys:0000_0000_700a_e000 = 0000_0000_7e05_5067 (P|RW|US|A|D)
phys:0000_0000_7e05_5030 = 0000_0001_66e2_9067 (P|RW|US|A|D)
phys:0000_0001_66e2_9000 = 0000_0001_69db_3067 (P|RW|US|A|D)
phys:0000_0001_69db_3000 = 0000_0000_8020_0225 (P|US|A|SOFTW1)

Am I doing something wrong?

Jethro

Mapping IO memory read-write

[Kenneth Adam Miller]

In this case, have you tried reading and writing to the memory segment
being mmap'd from userland?

Here's an example mmap'ing device driver if you need to see that:
https://github.com/claudioscordino/mmap_alloc

On Tue, Jan 12, 2016 at 6:10 PM, Jethro Beekman <kernel@jbeekman.nl> wrote:

> I'm writing a device driver for a memory-mapped device on x86-64. I'm
> mapping
> the device in the kernel using ioremap_cache(). My file_operations.mmap
> function
> is as follows:
>
> static int dev_mmap(struct file *filep, struct vm_area_struct *vma) {
>         vma->vm_page_prot.pgprot|=_PAGE_BIT_RW;
>         return vm_iomap_memory(vma,START,LEN);
> }
>
> The user process calls mmap(..,PROT_READ|PROT_WRITE|PROT_EXEC,..). The
> mapping
> in /proc/[pid]/maps shows the write bit. However, when looking at the
> actual
> page table entry does not have the RW bit set. For example:
>
> virtual address = 0000_0001_8000_0000
> cr3 = 0000_0000_700a_e000
> phys:0000_0000_700a_e000 = 0000_0000_7e05_5067 (P|RW|US|A|D)
> phys:0000_0000_7e05_5030 = 0000_0001_66e2_9067 (P|RW|US|A|D)
> phys:0000_0001_66e2_9000 = 0000_0001_69db_3067 (P|RW|US|A|D)
> phys:0000_0001_69db_3000 = 0000_0000_8020_0225 (P|US|A|SOFTW1)
>
> Am I doing something wrong?
>
> Jethro
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
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Mapping IO memory read-write

[Jethro Beekman]

> In this case, have you tried reading and writing to the memory segment being
> mmap'd from userland?
>
> Here's an example mmap'ing device driver if you need to see that:
> https://github.com/claudioscordino/mmap_alloc

That would not have worked, but your example code did give me an idea. I was
using MAP_PRIVATE while I should've been using MAP_SHARED. It works now. Thanks!

Jethro

On 13-01-16 06:34, Kenneth Adam Miller wrote:
> In this case, have you tried reading and writing to the memory segment being
> mmap'd from userland? 
> 
> Here's an example mmap'ing device driver if you need to see that:
> https://github.com/claudioscordino/mmap_alloc
> 
> On Tue, Jan 12, 2016 at 6:10 PM, Jethro Beekman <kernel@jbeekman.nl
> <mailto:kernel@jbeekman.nl>> wrote:
> 
>     I'm writing a device driver for a memory-mapped device on x86-64. I'm mapping
>     the device in the kernel using ioremap_cache(). My file_operations.mmap function
>     is as follows:
> 
>     static int dev_mmap(struct file *filep, struct vm_area_struct *vma) {
>             vma->vm_page_prot.pgprot|=_PAGE_BIT_RW;
>             return vm_iomap_memory(vma,START,LEN);
>     }
> 
>     The user process calls mmap(..,PROT_READ|PROT_WRITE|PROT_EXEC,..). The mapping
>     in /proc/[pid]/maps shows the write bit. However, when looking at the actual
>     page table entry does not have the RW bit set. For example:
> 
>     virtual address = 0000_0001_8000_0000
>     cr3 = 0000_0000_700a_e000
>     phys:0000_0000_700a_e000 = 0000_0000_7e05_5067 (P|RW|US|A|D)
>     phys:0000_0000_7e05_5030 = 0000_0001_66e2_9067 (P|RW|US|A|D)
>     phys:0000_0001_66e2_9000 = 0000_0001_69db_3067 (P|RW|US|A|D)
>     phys:0000_0001_69db_3000 = 0000_0000_8020_0225 (P|US|A|SOFTW1)
> 
>     Am I doing something wrong?
> 
>     Jethro
> 
>     _______________________________________________
>     Kernelnewbies mailing list
>     Kernelnewbies at kernelnewbies.org <mailto:Kernelnewbies@kernelnewbies.org>
>     http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> 
>