linux-kernel - Re: [PATCH v2 2/2] arm64: tlb: Use the TLBI RANGE feature in arm64

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Message-ID: <cda3d6bc-a7e2-5669-372c-3c34cc822e08@huawei.com>
Date:   Tue, 14 Jul 2020 21:51:42 +0800
From:   Zhenyu Ye <yezhenyu2@...wei.com>
To:     Catalin Marinas <catalin.marinas@....com>
CC:     <will@...nel.org>, <suzuki.poulose@....com>, <maz@...nel.org>,
        <steven.price@....com>, <guohanjun@...wei.com>, <olof@...om.net>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linux-mm@...ck.org>, <arm@...nel.org>, <xiexiangyou@...wei.com>,
        <prime.zeng@...ilicon.com>, <zhangshaokun@...ilicon.com>,
        <kuhn.chenqun@...wei.com>
Subject: Re: [PATCH v2 2/2] arm64: tlb: Use the TLBI RANGE feature in arm64

On 2020/7/14 18:36, Catalin Marinas wrote:
> On Fri, Jul 10, 2020 at 05:44:20PM +0800, Zhenyu Ye wrote:
>> +#define __TLBI_RANGE_PAGES(num, scale)	(((num) + 1) << (5 * (scale) + 1))
>> +#define MAX_TLBI_RANGE_PAGES		__TLBI_RANGE_PAGES(31, 3)
>> +
>> +#define TLBI_RANGE_MASK			GENMASK_ULL(4, 0)
>> +#define __TLBI_RANGE_NUM(range, scale)	\
>> +	(((range) >> (5 * (scale) + 1)) & TLBI_RANGE_MASK)
> [...]
>> +	int num = 0;
>> +	int scale = 0;
> [...]
>> +			start += __TLBI_RANGE_PAGES(num, scale) << PAGE_SHIFT;
> [...]
> 
> Since num is an int, __TLBI_RANGE_PAGES is still an int. Shifting it by
> PAGE_SHIFT can overflow as the maximum would be 8GB for 4K pages (or
> 128GB for 64K pages). I think we probably get away with this because of
> some implicit type conversion but I'd rather make __TLBI_RANGE_PAGES an
> explicit unsigned long:
> 
> #define __TLBI_RANGE_PAGES(num, scale)	((unsigned long)((num) + 1) << (5 * (scale) + 1))
> 

This is valuable and I will update this in next series, with the check
for binutils (or encode the instructions by hand), as soon as possible.

> Without this change, the CBMC check fails (see below for the test). In
> the kernel, we don't have this problem as we encode the address via
> __TLBI_VADDR_RANGE and it doesn't overflow.> The good part is that CBMC reckons the algorithm is correct ;).

Thanks for your test!

Zhenyu