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

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Date:	Fri, 22 Aug 2014 11:43:07 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Juergen Gross <JGross@...e.com>, stefan.bader@...onical.com,
	x86@...nel.org, ville.syrjala@...ux.intel.com,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	hpa@...or.com
Subject: Re: [Xen-devel] [PATCH RFC 1/3] x86: Make page cache mode a real
 type

On Fri, 2014-08-22 at 10:24 +0100, Jan Beulich wrote:
> >>> On 21.08.14 at 11:30, <JGross@...e.com> wrote:
> > On 08/20/2014 09:26 PM, Toshi Kani wrote:
> >> On Tue, 2014-08-19 at 15:25 +0200, jgross@...e.com wrote:
> >>> --- a/arch/x86/mm/init.c
> >>> +++ b/arch/x86/mm/init.c
> >>> @@ -27,6 +27,35 @@
> >>>
> >>>   #include "mm_internal.h"
> >>>
> >>> +/*
> >>> + * Tables translating between page_cache_type_t and pte encoding.
> >>> + * Minimal supported modes are defined statically, modified if more supported
> >>> + * cache modes are available.
> >>> + * Index into __cachemode2pte_tbl is the cachemode.
> >>> + * Index into __pte2cachemode_tbl are the caching attribute bits of the pte
> >>> + * (_PAGE_PWT, _PAGE_PCD, _PAGE_PAT) at index bit positions 0, 1, 2.
> >>> + */
> >>> +uint16_t __cachemode2pte_tbl[_PAGE_CACHE_MODE_NUM] = {
> >>> +	[_PAGE_CACHE_MODE_WB]		= 0,
> >>> +	[_PAGE_CACHE_MODE_WC]		= _PAGE_PWT,
> >>> +	[_PAGE_CACHE_MODE_UC_MINUS]	= _PAGE_PCD,
> >>> +	[_PAGE_CACHE_MODE_UC]		= _PAGE_PCD | _PAGE_PWT,
> >>> +	[_PAGE_CACHE_MODE_WT]		= _PAGE_PWT,
> >>> +	[_PAGE_CACHE_MODE_WP]		= _PAGE_PWT,
> >>> +};
> >>
> >> I think WT and WP should be set to _PAGE_PCD (UC_MINUS) for safe.
> > 
> > Oh, you are right.
> 
> Actually I suppose the original comment was about WC and WP;
> defaulting WT to _PAGE_PWT seems quite correct to me.

My comment was about WT and WP, whose cache modes are defined here but
are not supported in this patchset.  _PAGE_PWT is used by WC, which has
weakly ordered writes.  WT and WP have strongly ordered writes, so they
should be redirected to UC- for safe.

Thanks,
-Toshi

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