linux-kernel - Re: [PATCH v3 2/2] x86/sgx: Add SGX

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Date:   Fri, 03 Sep 2021 01:14:03 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Kai Huang <kai.huang@...el.com>
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>,
        Jonathan Corbet <corbet@....net>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 2/2] x86/sgx: Add SGX_MemTotal to /proc/meminfo

On Fri, 2021-09-03 at 09:56 +1200, Kai Huang wrote:
> On Thu, 02 Sep 2021 15:15:51 +0300 Jarkko Sakkinen wrote:
> > On Wed, 2021-09-01 at 17:47 +1200, Kai Huang wrote:
> > > On Wed, 01 Sep 2021 08:41:12 +0300 Jarkko Sakkinen wrote:
> > > > On Wed, 2021-09-01 at 17:33 +1200, Kai Huang wrote:
> > > > > On Wed, 01 Sep 2021 05:02:45 +0300 Jarkko Sakkinen wrote:
> > > > > > On Sat, 2021-08-28 at 00:03 +1200, Kai Huang wrote:
> > > > > > > > > > -/* The free page list lock protected variables prepend the lock. */
> > > > > > > > > > +/* The number of usable EPC pages in the system. */
> > > > > > > > > > +unsigned long sgx_nr_all_pages;
> > > > > > > > > > +
> > > > > > > > > > +/* The number of free EPC pages in all nodes. */
> > > > > > > > > >  static unsigned long sgx_nr_free_pages;
> > > > > > > > > >  
> > > > > > > > > >  /* Nodes with one or more EPC sections. */
> > > > > > > > > > @@ -656,6 +659,8 @@ static bool __init sgx_setup_epc_section(u64 phys_addr, u64 size,
> > > > > > > > > >  		list_add_tail(&section->pages[i].list, &sgx_dirty_page_list);
> > > > > > > > > >  	}
> > > > > > > > > >  
> > > > > > > > > > +	sgx_nr_all_pages += nr_pages;
> > > > > > > > > > +
> > > > > > > > > 
> > > > > > > > > EPC sections can be freed again in sgx_init() after they are successfully
> > > > > > > > > initialized, when any further initialization fails (i.e. when fails to create
> > > > > > > > > ksgxd, or fails to register /dev/sgx_provision).  In which case, I think
> > > > > > > > > sgx_nr_all_pages should also be cleared.  But current sgx_init() seems doesn't
> > > > > > > > > reset it.  Do you need to fix that too?
> > > > > > > > 
> > > > > > > > sgx_nr_all_pages tells just the total pages in the system, i.e. it's a constant.
> > > > > > > > 
> > > > > > > > Maybe a rename to "sgx_nr_total_pages" would be a good idea? Would match with
> > > > > > > > the meminfo field better too.
> > > > > > > 
> > > > > > > I don't have preference on name.  I just think if there's no actual user of
> > > > > > > EPC (when both driver and KVM SGX cannot be enabled), it's pointless to print
> > > > > > > number of EPC pages.
> > > > > > 
> > > > > > I'd presume that you refer to the code, which prints the number of *bytes* in
> > > > > > the system because code printing the number of pages does not exist in this
> > > > > > patch set.
> > > > > > 
> > > > > > I have troubles the decipher your statement.
> > > > > > 
> > > > > > You think that only if both the driver and KVM are *both* enabled, only then
> > > > > > it makes sense to have this information available for sysadmin?
> > > > > 
> > > > > Only if at least one of them is enabled.
> > > > 
> > > > OK, thank you, that does make sense.
> > > > 
> > > > What would happen if neither is enabled is that SGX_MemTotal would
> > > > state that there is zero bytes of EPC. 
> > > 
> > > This is the problem I pointed out at the beginning, that (if I read code
> > > correctly), it seems your current patch doesn't clear sgx_nr_all_pages when
> > > neither is enabled (in sgx_init() in sgx/main.c).
> > 
> > It's initialized to zero, so are you talking about fallback when something
> > fails?
> > 
> > /Jarkko
> 
> Yes, shouldn't you have something similar to below?
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 63d3de02bbcc..270f6103b6c0 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -836,6 +836,7 @@ static int __init sgx_init(void)
>                 vfree(sgx_epc_sections[i].pages);
>                 memunmap(sgx_epc_sections[i].virt_addr);
>         }
> +       sgx_nr_all_pages = 0;
>  
>         return ret;
>  }

Yeah, something along the lines. Thanks, this was really good
remark. The fallback path is clearly broken.

/Jarkko