Re: [RFC PATCH v3 08/27] x86/sgx: Initialize virtual EPC driver even when SGX driver is disabled

From: Jarkko Sakkinen <jarkko@kernel.org>
To: Kai Huang <kai.huang@intel.com>
Cc: Sean Christopherson <seanjc@google.com>,
	Dave Hansen <dave.hansen@intel.com>,
	linux-sgx@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org,
	luto@kernel.org, haitao.huang@intel.com, pbonzini@redhat.com,
	bp@alien8.de, tglx@linutronix.de, mingo@redhat.com,
	hpa@zytor.com
Subject: Re: [RFC PATCH v3 08/27] x86/sgx: Initialize virtual EPC driver even when SGX driver is disabled
Date: Wed, 3 Feb 2021 01:10:50 +0200	[thread overview]
Message-ID: <YBnb+hTkHqmuUeX3@kernel.org> (raw)
In-Reply-To: <20210202131207.a806a0c35140a9a210aaccec@intel.com>

On Tue, Feb 02, 2021 at 01:12:07PM +1300, Kai Huang wrote:
> On Mon, 1 Feb 2021 09:23:18 -0800 Sean Christopherson wrote:
> > On Mon, Feb 01, 2021, Dave Hansen wrote:
> > > On 1/31/21 9:40 PM, Kai Huang wrote:
> > > >>> -	ret = sgx_drv_init();
> > > >>> +	/* Success if the native *or* virtual EPC driver initialized cleanly. */
> > > >>> +	ret = !!sgx_drv_init() & !!sgx_vepc_init();
> > > >> If would create more dumb code and just add
> > > >>
> > > >> ret = sgx_vepc_init()
> > > >> if (ret)
> > > >>         goto err_kthread;
> > > 
> > > Jarkko, I'm not sure I understand this suggestion.
> > > 
> > > > Do you mean you want below?
> > > > 
> > > > 	ret = sgx_drv_init();
> > > > 	ret = sgx_vepc_init();
> > > > 	if (ret)
> > > > 		goto err_kthread;
> > > > 
> > > > This was Sean's original code, but Dave didn't like it.
> > 
> > The problem is it's wrong.  That snippet would incorrectly bail if drv_init()
> > succeeds but vepc_init() fails.
> > 
> > The alternative to the bitwise AND is to snapshot the result in two separate
> > variables:
> > 
> > 	ret = sgx_drv_init();
> > 	ret2 = sgx_vepc_init();
> > 	if (ret && ret2)
> > 		goto err_kthread;
> > 
> > or check the return from drv_init() _after_ vepc_init():
> > 
> > 	ret = sgx_drv_init();
> > 	if (sgx_vepc_init() && ret)
> > 		goto err_kthread;
> > 
> > 
> > As evidenced by this thread, the behavior is subtle and easy to get wrong.  I
> > deliberately chose the option that was the weirdest specifically to reduce the
> > probability of someone incorrectly "cleaning up" the code.
> > 
> > > Are you sure?  I remember the !!&!! abomination being Sean's doing. :)
> > 
> > Yep!  That 100% functionally correct horror is my doing.
> > 
> > > > Sean/Dave,
> > > > 
> > > > Please let me know which way you prefer.
> > > 
> > > Kai, I don't really know you are saying here.  In the end,
> > > sgx_vepc_init() has to run regardless of whether sgx_drv_init() is
> > > successful or not.  Also, we only want to 'goto err_kthraed' if *BOTH*
> > > fail.  The code you have above will, for instance, 'goto err_kthread' if
> > > sgx_drv_init() succeeds but sgx_vepc_init() fails.  It entirely
> > > disregards the sgx_drv_init() error code.
> > 
> 
> Hi Dave, Sean,
> 
> Yeah sorry my bad. The example I provided won't work. So I'd like keep the !!
> &!! :)
> 
> Thanks. 
> 
> Jarkko, please let us know if you still have concern.

I'll just review the next version. I disagree with this packing though.

/Jarkko