Re: [Xen-devel] [PATCH 1/3] x86: Make page cache mode a real type

From: Juergen Gross
Date: Wed Aug 27 2014 - 00:08:00 EST

On 08/26/2014 09:44 PM, Toshi Kani wrote:
On Tue, 2014-08-26 at 08:16 +0200, Juergen Gross wrote:
At the moment there are a lot of places that handle setting or getting
the page cache mode by treating the pgprot bits equal to the cache mode.
This is only true because there are a lot of assumptions about the setup
of the PAT MSR. Otherwise the cache type needs to get translated into
pgprot bits and vice versa.

This patch tries to prepare for that by introducing a seperate type
for the cache mode and adding functions to translate between those and pgprot

To avoid too much performance penalty the translation between cache mode
and pgprot values is done via tables which contain the relevant information.
Write-back cache mode is hard-wired to be 0, all other modes are configurable
via those tables. For large pages there are translation functions as the
PAT bit is located at different positions in the ptes of 4k and large pages.

Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Hi Juergen,

Thanks for the updates! A few comments below...

@@ -73,6 +73,9 @@ void *kmap_atomic_prot_pfn(unsigned long pfn, pgprot_t prot)
* Map 'pfn' using protections 'prot'
+#define __PAGE_KERNEL_WC (__PAGE_KERNEL | \
+ cachemode2protval(_PAGE_CACHE_MODE_WC))
void __iomem *
iomap_atomic_prot_pfn(unsigned long pfn, pgprot_t prot)
@@ -82,12 +85,14 @@ iomap_atomic_prot_pfn(unsigned long pfn, pgprot_t prot)
* MTRR is UC or WC. UC_MINUS gets the real intention, of the
* user, which is "WC if the MTRR is WC, UC if you can't do that."
- if (!pat_enabled && pgprot_val(prot) == pgprot_val(PAGE_KERNEL_WC))
+ if (!pat_enabled && pgprot_val(prot) == __PAGE_KERNEL_WC)
+ prot = __pgprot(__PAGE_KERNEL |
+ protval_pagemode(_PAGE_CACHE_MODE_UC_MINUS));

protval_pagemode() should be cachemode2protval().

Obviously, yes.

diff --git a/drivers/video/fbdev/vermilion/vermilion.c b/drivers/video/fbdev/vermilion/vermilion.c
index 048a666..6bbc559 100644
--- a/drivers/video/fbdev/vermilion/vermilion.c
+++ b/drivers/video/fbdev/vermilion/vermilion.c
@@ -1004,13 +1004,15 @@ static int vmlfb_mmap(struct fb_info *info, struct vm_area_struct *vma)
struct vml_info *vinfo = container_of(info, struct vml_info, info);
unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
int ret;
+ unsigned long prot;

ret = vmlfb_vram_offset(vinfo, offset);
if (ret)
return -EINVAL;

- pgprot_val(vma->vm_page_prot) |= _PAGE_PCD;
- pgprot_val(vma->vm_page_prot) &= ~_PAGE_PWT;
+ prot = pgprot_val(vma->vm_page_prot) & ~_PAGE_CACHE_MASK;
+ pgprot_val(vma->vm_page_prot) =
+ prot | cachemode2protval(_PAGE_CACHE_MODE_UC);

This cache mode should be _PAGE_CACHE_MODE_UC_MINUS as the original code
only sets the PCD bit.

I'll change it.



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