Userspace pages in UC mode

[Sabela Ramos Garea]

Sorry, little mistake copypasting and cleaning. The pages and vma
structs should look like that:

struct page *pages --> struct page *pages[MAX_PAGES];
struct vma_area_struct *vma --> struct vma_area_struct *vma[MAX_PAGES];

Where MAX_PAGES is defined to 5.

Sabela.

2015-09-11 16:07 GMT+02:00 Sabela Ramos Garea <sabelaraga at gmail.com>:
> Dear all,
>
> For research purposes I need some userspace memory pages to be in
> uncacheable mode. I am using two different Intel architectures (Sandy
> Bridge and Haswell) and two different kernels (2.6.32-358 and
> 3.19.0-28).
>
> The non-temporal stores from Intel assembly are not a valid solution
> so I am programming a kernel module that gets a set of pages from user
> space reserved with posix_memalign (get_user_pages) and then sets them
> as uncacheable (I have tried set_pages_uc and set_pages_array_uc).
> When I use one page, the access times are not very coherent and with
> more than one page the module crashes (in both architectures and both
> kernels).
>
> I wonder if I am using the correct approach or if I have to use kernel
> space pages in order to work with uncacheable memory. Or if I have to
> remap the memory. Just in case it makes it clearer, I am attaching the
> relevant lines of a kernel module function that should set the pages
> as uncacheable. (This function is the .write of a misc device; count
> is treated as the number of pages).
>
> Best and Thanks,
>
> Sabela.
>
> struct page *pages; //defined outside in order to be able to set them
> to WB in the release function.
> int numpages;
>
> static ssize_t setup_memory(struct file *filp, const char __user *buf,
> size_t count, loff_t * ppos)
> {
>         int res;
>         struct vm_area_struct *vmas;
>
>         numpages = count/4096;
>
>         down_read(&current->mm->mmap_sem);
>         res = get_user_pages(current, current->mm,
>                                 (unsigned long) buf,
>                                 numpages, /* Number of pages */
>                                 0, /* Do want to write into it */
>                                 1, /* do force */
>                                 &pages,
>                                 &vmas);
>         up_read(&current->mm->mmap_sem);
>
>         numpages=res;
>
>         if (res > 0) {
>                 set_pages_uc(pages, numpages); /* Uncached */
>                 printk("Write: %d pages set as uncacheable\n",numpages);
>         }
>         else{
>                 pr_err("Couldn't get pages to set them as UC :(\n");
>                 return -EAGAIN;
>         }
> }