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

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Date:	Wed, 27 Aug 2014 06:07:52 +0200
From:	Juergen Gross <jgross@...e.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	xen-devel@...ts.xensource.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, stefan.bader@...onical.com,
	hpa@...or.com, ville.syrjala@...ux.intel.com
Subject: Re: [Xen-devel] [PATCH 1/3] x86: Make page cache mode a real type

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
>> values.
>>
>> 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@...onical.com>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>
> 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))
>> -		prot = PAGE_KERNEL_UC_MINUS;
>> +	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.

Thanks,

Juergen

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