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Date:   Wed, 10 Feb 2021 12:17:23 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Mukesh Ojha <mojha@codeaurora.org>
Cc:     linux-kernel@vger.kernel.org, tony.luck@intel.com,
        ccross@android.com, anton@enomsg.org,
        Huang Yiwei <hyiwei@codeaurora.org>
Subject: Re: [PATCH] pstore/ram : Add support for cached pages
Message-ID: <202102101213.D603669D4@keescook>
References: <1612968741-1692-1-git-send-email-mojha@codeaurora.org>
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On Wed, Feb 10, 2021 at 08:22:21PM +0530, Mukesh Ojha wrote:
> There could be a sceanario where we define some region
> in normal memory and use them store to logs which is later
> retrieved by bootloader during warm reset.
> 
> In this scenario, we wanted to treat this memory as normal
> cacheable memory instead of default behaviour which
> is an overhead. Making it cacheable could improve
> performance.

Cool; yeah. I like this idea.

> 
> This commit gives control to change mem_type from Device
> tree, and also documents the value for normal memory.

What's the safest default setting?

> 
> Signed-off-by: Huang Yiwei <hyiwei@codeaurora.org>
> Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>
> ---
>  Documentation/admin-guide/ramoops.rst |  4 +++-
>  fs/pstore/ram.c                       |  1 +
>  fs/pstore/ram_core.c                  | 10 ++++++++--
>  3 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/ramoops.rst b/Documentation/admin-guide/ramoops.rst
> index b0a1ae7..8f107d8 100644
> --- a/Documentation/admin-guide/ramoops.rst
> +++ b/Documentation/admin-guide/ramoops.rst
> @@ -3,7 +3,7 @@ Ramoops oops/panic logger
>  
>  Sergiu Iordache <sergiu@chromium.org>
>  
> -Updated: 17 November 2011
> +Updated: 10 Feb 2021
>  
>  Introduction
>  ------------
> @@ -30,6 +30,8 @@ mapping to pgprot_writecombine. Setting ``mem_type=1`` attempts to use
>  depends on atomic operations. At least on ARM, pgprot_noncached causes the
>  memory to be mapped strongly ordered, and atomic operations on strongly ordered
>  memory are implementation defined, and won't work on many ARMs such as omaps.
> +Setting ``mem_type=2`` attempts to treat the memory region as normal memory,
> +which enables full cache on it. This can improve the performance.
>  
>  The memory area is divided into ``record_size`` chunks (also rounded down to
>  power of two) and each kmesg dump writes a ``record_size`` chunk of
> diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
> index ca6d8a8..b262c57 100644
> --- a/fs/pstore/ram.c
> +++ b/fs/pstore/ram.c
> @@ -666,6 +666,7 @@ static int ramoops_parse_dt(struct platform_device *pdev,
>  		field = value;						\
>  	}
>  
> +	parse_u32("mem-type", pdata->record_size, pdata->mem_type);

This was handled by "unbuffered" above. Can you find a way to resolve
potential conflicts between these?

>  	parse_u32("record-size", pdata->record_size, 0);
>  	parse_u32("console-size", pdata->console_size, 0);
>  	parse_u32("ftrace-size", pdata->ftrace_size, 0);
> diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
> index aa8e0b6..83cd612 100644
> --- a/fs/pstore/ram_core.c
> +++ b/fs/pstore/ram_core.c
> @@ -396,6 +396,10 @@ void persistent_ram_zap(struct persistent_ram_zone *prz)
>  	persistent_ram_update_header_ecc(prz);
>  }
>  
> +#define MEM_TYPE_WCOMBINE	0
> +#define MEM_TYPE_NONCACHED	1
> +#define MEM_TYPE_NORMAL		2

It might be nice for this to have a human-readable name too, but let's
start with numeric. :)

Please update the mem_type MODULE_PARM_DESC to detail the new values too.

> +
>  static void *persistent_ram_vmap(phys_addr_t start, size_t size,
>  		unsigned int memtype)
>  {
> @@ -409,9 +413,11 @@ static void *persistent_ram_vmap(phys_addr_t start, size_t size,
>  	page_start = start - offset_in_page(start);
>  	page_count = DIV_ROUND_UP(size + offset_in_page(start), PAGE_SIZE);
>  
> -	if (memtype)
> +	if (memtype == MEM_TYPE_NORMAL)
> +		prot = PAGE_KERNEL;
> +	else if (memtype == MEM_TYPE_NONCACHED)
>  		prot = pgprot_noncached(PAGE_KERNEL);
> -	else
> +	else if (memtype == MEM_TYPE_WCOMBINE)
>  		prot = pgprot_writecombine(PAGE_KERNEL);

Let's make this a switch statement.

>  
>  	pages = kmalloc_array(page_count, sizeof(struct page *), GFP_KERNEL);
> -- 
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center,
> Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
> 

-- 
Kees Cook