linux-kernel - Re: [PATCH v4 2/7] x86, mm, pat: Change reserve

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Date:	Mon, 03 Nov 2014 11:57:29 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	hpa@...or.com, mingo@...hat.com, akpm@...ux-foundation.org,
	arnd@...db.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	jgross@...e.com, stefan.bader@...onical.com, luto@...capital.net,
	hmh@....eng.br, yigal@...xistor.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v4 2/7] x86, mm, pat: Change reserve_memtype() to handle
 WT

On Mon, 2014-11-03 at 19:27 +0100, Thomas Gleixner wrote:
> On Mon, 27 Oct 2014, Toshi Kani wrote:
> > This patch changes reserve_memtype() to handle the WT cache mode.
> > When PAT is not enabled, it continues to set UC- to *new_type for
> > any non-WB request.
> > 
> > When a target range is RAM, reserve_ram_pages_type() fails for WT
> > for now.  This function may not reserve a RAM range for WT since
> > reserve_ram_pages_type() uses the page flags limited to three memory
> > types, WB, WC and UC.
> > 
> > Signed-off-by: Toshi Kani <toshi.kani@...com>
> > Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > ---
> >  arch/x86/include/asm/cacheflush.h |    4 ++++
> >  arch/x86/mm/pat.c                 |   16 +++++++++++++---
> >  2 files changed, 17 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/cacheflush.h b/arch/x86/include/asm/cacheflush.h
> > index 157644b..c912680 100644
> > --- a/arch/x86/include/asm/cacheflush.h
> > +++ b/arch/x86/include/asm/cacheflush.h
> > @@ -53,6 +53,10 @@ static inline void set_page_memtype(struct page *pg,
> >  	case _PAGE_CACHE_MODE_WB:
> >  		memtype_flags = _PGMT_WB;
> >  		break;
> > +	case _PAGE_CACHE_MODE_WT:
> > +	case _PAGE_CACHE_MODE_WP:
> > +		pr_err("set_page_memtype: unsupported cachemode %d\n", memtype);
> > +		BUG();
> 
> You already catch the cases with the hunk below at the entry of
> reserve_ram_pages_type(). So what's the point of the BUG()?
> 
> If you are worried about other usage sites: This function should not
> at all be in arch/x86/include/asm/cacheflush.h. It's solely used by
> PAT, so we really should move it there before changing it.

Yes, I was worried about other usage.  I agree that this function should
belong to PAT.  I will move it to pat.c before changing it, and will
remove BUG() from this patch. 

> >  	default:
> >  		memtype_flags = _PGMT_DEFAULT;
> >  		break;
> > diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> > index db687c3..a214f5a 100644
> > --- a/arch/x86/mm/pat.c
> > +++ b/arch/x86/mm/pat.c
> > @@ -289,6 +289,8 @@ static int pat_pagerange_is_ram(resource_size_t start, resource_size_t end)
> >  
> >  /*
> >   * For RAM pages, we use page flags to mark the pages with appropriate type.
> > + * The page flags are currently limited to three types, WB, WC and UC. Hence,
> > + * any request to WT or WP will fail with -EINVAL.
> >   * Here we do two pass:
> >   * - Find the memtype of all the pages in the range, look for any conflicts
> >   * - In case of no conflicts, set the new memtype for pages in the range
> > @@ -300,6 +302,13 @@ static int reserve_ram_pages_type(u64 start, u64 end,
> >  	struct page *page;
> >  	u64 pfn;
> >  
> > +	if ((req_type == _PAGE_CACHE_MODE_WT) ||
> > +	    (req_type == _PAGE_CACHE_MODE_WP)) {
> > +		if (new_type)
> > +			*new_type = _PAGE_CACHE_MODE_UC_MINUS;
> > +		return -EINVAL;
> > +	}
> > +
> >  	if (req_type == _PAGE_CACHE_MODE_UC) {
> >  		/* We do not support strong UC */
> >  		WARN_ON_ONCE(1);
> > @@ -349,6 +358,7 @@ static int free_ram_pages_type(u64 start, u64 end)
> >   * - _PAGE_CACHE_MODE_WC
> >   * - _PAGE_CACHE_MODE_UC_MINUS
> >   * - _PAGE_CACHE_MODE_UC
> > + * - _PAGE_CACHE_MODE_WT
> >   *
> >   * If new_type is NULL, function will return an error if it cannot reserve the
> >   * region with req_type. If new_type is non-NULL, function will return
> > @@ -368,10 +378,10 @@ int reserve_memtype(u64 start, u64 end, enum page_cache_mode req_type,
> >  	if (!pat_enabled) {
> >  		/* This is identical to page table setting without PAT */
> >  		if (new_type) {
> > -			if (req_type == _PAGE_CACHE_MODE_WC)
> > -				*new_type = _PAGE_CACHE_MODE_UC_MINUS;
> > +			if (req_type == _PAGE_CACHE_MODE_WB)
> > +				*new_type = _PAGE_CACHE_MODE_WB;
> >  			else
> > -				*new_type = req_type;
> > +				*new_type = _PAGE_CACHE_MODE_UC_MINUS;
> 
> So until now we supported WB, UC- and UC and mapped WC to UC-. Now we
> map everything except WB to UC- 
> Why feels that wrong without a comment explaining it?

The case of !pat_enable only supports WB and UC-.  Previously, WC was
the only type that needed to be mapped to UC-.  The code now handles it
properly for any other types.  Yes, I will add a comment for this. 

Thanks,
-Toshi

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