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

[Tianjia Zhang]

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@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.

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