linux-kernel - Re: [PATCH v2] arm64: cache: Skip an unnecessary data cache clean PoU operation

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Date:   Tue, 21 Feb 2017 15:49:34 +0000
From:   Will Deacon <will.deacon@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Shanker Donthineni <shankerd@...eaurora.org>,
        Mark Rutland <mark.rutland@....com>,
        Vikram Sethi <vikrams@...eaurora.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        James Morse <james.morse@....com>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] arm64: cache: Skip an unnecessary data cache clean
 PoU operation

On Tue, Feb 21, 2017 at 03:47:27PM +0000, Catalin Marinas wrote:
> On Wed, Feb 08, 2017 at 03:19:37PM -0600, Shanker Donthineni wrote:
> > The cache management functions always do the data cache PoU
> > (point of unification) operations even though it is not required
> > on some systems. No need to clean data cache till PoU if all the
> > cache levels below PoUIS are WT (Write-Through) caches. It causes
> > a huge performance degradation when operating on a larger memory
> > area, especially THP with 64K page size kernel.
> > 
> > For each online CPU, check the need of 'dc cvau' instruction and
> > update a global variable __dcache_flags. The two functions
> > __flush_cache_user_range() and __clean_dcache_area_pou() are
> > modified to skip an unnecessary code execution based on flags.
> > It won't change the existing behavior if any one of the online
> > CPU is capable of WB cache below PoUIS level.
> > 
> > Signed-off-by: Shanker Donthineni <shankerd@...eaurora.org>
> [...]
> > +#define CLIDR_LOUIS_SHIFT	(21)
> > +#define CLIDR_LOUIS_MASK	(0x7)
> > +#define CLIDR_LOUIS(x)		(((x) >> CLIDR_LOUIS_SHIFT) & CLIDR_LOUIS_MASK)
> 
> According to the ARMv8 ARM, CLIDR_EL1 "identifies the type of cache, or
> caches, that are implemented at each level and can be managed using the
> architected cache maintenance instructions that operate by set/way". The
> key part is "set/way" here and hence you cannot use CLIDR_EL1 and
> CCSIDR_EL1 to infer whether you can skip cache maintenance by VA.
> 
> > +	/* Go through all the cache level below LoUIS */
> > +	for (lvl = 0; lvl < louis; lvl++) {
> > +		csidr = cache_get_ccsidr(lvl << 1);
> > +		if (csidr & CCSIDR_EL1_WRITE_BACK) {
> 
> The type bits have also been deprecated in ARMv8 (we need to update the
> kernel or just remove the cache topology detection entirely, leaving it
> just to DT).

I'll dust off the patches I have for this...

Will