Re: [PATCH v3 3/5] x86/sgx: Optimize the free_cnt count in sgx_epc_section

[Jarkko Sakkinen]

I could bet some money that this does not bring any significant
performance gain.

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@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  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.

>  		} else
>  			list_move_tail(&page->list, &dirty);
> @@ -643,7 +644,6 @@ static bool __init sgx_setup_epc_section(u64 phys_addr, u64 size,
>  		list_add_tail(&section->pages[i].list, &section->init_laundry_list);
>  	}
>  
> -	section->free_cnt = nr_pages;
>  	return true;
>  }
>  
> -- 
> 2.19.1.3.ge56e4f7
> 
> 

/Jarkko