linux-kernel - Re: [PATCH v3 3/5] x86/sgx: Optimize the free_cnt count in sgx_epc

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Date:   Thu, 11 Feb 2021 14:04:12 +0800
From:   Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Shuah Khan <shuah@...nel.org>, x86@...nel.org,
        linux-sgx@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Jia Zhang <zhang.jia@...ux.alibaba.com>
Subject: Re: [PATCH v3 3/5] x86/sgx: Optimize the free_cnt count in
 sgx_epc_section

Hi,

Sorry for the late reply.

On 1/28/21 1:40 AM, Jarkko Sakkinen wrote:
> I could bet some money that this does not bring any significant
> performance gain.
> 

Yes, this does not bring performance gains. This is not a change for 
performance, mainly to make the value of free_cnt look more accurate.

> On Sun, Jan 24, 2021 at 02:29:05PM +0800, Tianjia Zhang wrote:
>> `section->free_cnt` represents the free page in sgx_epc_section,
>> which is assigned once after initialization. In fact, just after the
>> initialization is completed, the pages are in the `init_laundry_list`
>> list and cannot be allocated. This needs to be recovered by EREMOVE
>> of function sgx_sanitize_section() before it can be used as a page
>> that can be allocated. The sgx_sanitize_section() will be called in
>> the kernel thread ksgxd.
>>
>> This patch moves the initialization of `section->free_cnt` from the
>> initialization function `sgx_setup_epc_section()` to the function
>> `sgx_sanitize_section()`, and then accumulates the count after the
> 
> Use single quotes instead of hyphens.
> >> successful execution of EREMOVE. This seems to be more reasonable,
>> free_cnt will also truly reflect the allocatable free pages in EPC.
>>
>> Sined-off-by: Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
>> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
>> ---
>>   arch/x86/kernel/cpu/sgx/main.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
>> index 4465912174fd..e455ec7b3449 100644
>> --- a/arch/x86/kernel/cpu/sgx/main.c
>> +++ b/arch/x86/kernel/cpu/sgx/main.c
>> @@ -48,6 +48,7 @@ static void sgx_sanitize_section(struct sgx_epc_section *section)
>>   		if (!ret) {
>>   			spin_lock(&section->lock);
>>   			list_move(&page->list, &section->page_list);
>> +			section->free_cnt++;
>>   			spin_unlock(&section->lock);
> 
> Someone can try to allocate a page while sanitize process is in progress.
> 
> I think it is better to keep critical sections in the form that when you
> leave from one, the global state is legit.
> 

Do you mean to move the critical section to protect the entire while 
loop? Of course, this is also possible, sanitize is a process only 
needed for initialization, and the possibility of conflict is very small.

Best regards,
Tianjia