linux-kernel - Re: [PATCH v4 2/3] x86/sgx: Replace section local dirty page lists with a global list

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Date:   Mon, 15 Mar 2021 12:47:29 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     linux-sgx@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] x86/sgx: Replace section local dirty page lists
 with a global list

On 3/15/21 12:14 PM, Jarkko Sakkinen wrote:
> On Mon, Mar 15, 2021 at 09:03:21AM -0700, Dave Hansen wrote:
>> On 3/13/21 8:01 AM, Jarkko Sakkinen wrote:
>>> Reset initialized EPC pages in sgx_dirty_page_list to uninitialized state,
>>> and free them using sgx_free_epc_page(). Do two passes, as for SECS pages
>>> the first round can fail, if all child pages have not yet been removed.
>>> The driver puts all pages on startup first to sgx_dirty_page_list, as the
>>> initialization could be triggered by kexec(), meaning that pages have been
>>> reserved for active enclaves before the operation.
>>>
>>> The section local lists are redundant, as sgx_free_epc_page() figures
>>> out the correction by using epc_page->section.
>>
>> During normal runtime, the "ksgxd" daemon behaves like a  version of
>> kswapd just for SGX.  But, its first job is to initialize enclave
>> memory.  This is done in a a separate thread because this initialization
>> can be quite slow.
>>
>> Currently, the SGX boot code places each enclave page on a
>> sgx_section-local list (init_laundry_list).  Once it starts up, the
>> ksgxd code walks over that list and populates the actual SGX page allocator.
>>
>> However, the per-section structures are going away to make way for the
>> SGX NUMA allocator.  There's also little need to have a per-section
>> structure; the enclave pages are all treated identically, and they can
>> be placed on the correct allocator list from metadata stoered in the
>> enclave page itself.
>>
> Is this a suggestion how to rephrase the commit message? :-)

Yep.

>>>  static int ksgxd(void *p)
>>>  {
>>> -	int i;
>>> +	struct sgx_epc_page *page;
>>> +	LIST_HEAD(dirty);
>>> +	int i, ret;
>>>  
>>>  	set_freezable();
>>>  
>>>  	/*
>>> -	 * Sanitize pages in order to recover from kexec(). The 2nd pass is
>>> -	 * required for SECS pages, whose child pages blocked EREMOVE.
>>> +	 * Reset initialized EPC pages in sgx_dirty_page_list to uninitialized state,
>>> +	 * and free them using sgx_free_epc_page(). 
>>
>> I'm not a fan of comments that tell us verbatim what the code does.
> 
> So, what are you suggesting? Remove the whole comment or parts of it? I'm
> presuming that you suggest removing the "top-level" part.

Just please make it more relevant and informative.  I can tell this is
processing 'sgx_dirty_page_list' and calling sgx_free_epc_page() from
the code.  I don't need a comment to tell me that.

A better comment would say:

	Take enclave pages from the init code, ensure they are clean,
	and free the back into the main SGX page allocator

That tells what *role* this code plays (connecting init code to the
allocator) in a way that just verbatim telling what code is called does not.