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

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Date:   Tue, 21 Feb 2017 15:47:27 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Shanker Donthineni <shankerd@...eaurora.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Vikram Sethi <vikrams@...eaurora.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Will Deacon <will.deacon@....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 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).

-- 
Catalin