linux-kernel - Re: [PATCH 1/4] MIPS: Flush dcache for flush_kernel_dcache

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Date:	Tue, 29 Mar 2016 10:29:12 +0200
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Lars Persson <lars.persson@...s.com>
Cc:	Paul Burton <paul.burton@...tec.com>,
	"Steven J. Hill" <sjhill@...litydiluted.com>,
	"linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] MIPS: Flush dcache for flush_kernel_dcache_page

On Fri, Mar 04, 2016 at 03:09:00PM +0000, Lars Persson wrote:

> > 1 mars 2016 kl. 03:38 Paul Burton <paul.burton@...tec.com>:
> > 
> > The flush_kernel_dcache_page function was previously essentially a nop.
> > This is incorrect for MIPS, where if a page has been modified & either
> > it aliases or it's executable & the icache doesn't fill from dcache then
> > the content needs to be written back from dcache to the next level of
> > the cache hierarchy (which is shared with the icache).
> > 
> > Implement this by simply calling flush_dcache_page, treating this
> > kmapped cache flush function (flush_kernel_dcache_page) exactly the same
> > as its non-kmapped counterpart (flush_dcache_page).
> > 
> > Signed-off-by: Paul Burton <paul.burton@...tec.com>
> > ---
> > 
> > arch/mips/include/asm/cacheflush.h | 1 +
> > 1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/mips/include/asm/cacheflush.h b/arch/mips/include/asm/cacheflush.h
> > index 723229f..7e9f468 100644
> > --- a/arch/mips/include/asm/cacheflush.h
> > +++ b/arch/mips/include/asm/cacheflush.h
> > @@ -132,6 +132,7 @@ static inline void kunmap_noncoherent(void)
> > static inline void flush_kernel_dcache_page(struct page *page)
> > {
> >    BUG_ON(cpu_has_dc_aliases && PageHighMem(page));
> > +    flush_dcache_page(page);
> 
> Should we use instead __flush_dcache_page() that flushes immediately for mapped pages ? Steven J Hill's old patch set for highmem had done it like this.

Delayed flushing should be ok for lowmem where each page has a permanent
virtual address.  With highmem the temporary address assigned by the kmap_*
function may change so the flush needs to be performed immediately.
Special case highmem without cache aliases - the exact virtual address
doesn't matter, so this should be fine.

Cache flushes are expensive so delaying if possible is always a good thing.

Steven's patches afair were trying to tackle the highmem with aliases case
so an immediately flush was required.

  Ralf