linux-kernel - Re: [PATCH v22 07/24] x86/sgx: Add wrappers for ENCLS leaf functions

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Date:   Mon, 7 Oct 2019 21:04:05 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, nhorman@...hat.com, npmccallum@...hat.com,
        serge.ayoun@...el.com, shay.katz-zamir@...el.com,
        haitao.huang@...el.com, andriy.shevchenko@...ux.intel.com,
        tglx@...utronix.de, kai.svahn@...el.com, josh@...htriplett.org,
        luto@...nel.org, kai.huang@...el.com, rientjes@...gle.com,
        cedric.xing@...el.com
Subject: Re: [PATCH v22 07/24] x86/sgx: Add wrappers for ENCLS leaf functions

On Fri, Oct 04, 2019 at 11:45:13AM +0200, Borislav Petkov wrote:
> On Tue, Sep 03, 2019 at 05:26:38PM +0300, Jarkko Sakkinen wrote:
> > +/**
> > + * ENCLS_FAULT_FLAG - flag signifying an ENCLS return code is a trapnr
> > + *
> > + * ENCLS has its own (positive value) error codes and also generates
> > + * ENCLS specific #GP and #PF faults.  And the ENCLS values get munged
> > + * with system error codes as everything percolates back up the stack.
> > + * Unfortunately (for us), we need to precisely identify each unique
> > + * error code, e.g. the action taken if EWB fails varies based on the
> > + * type of fault and on the exact SGX error code, i.e. we can't simply
> > + * convert all faults to -EFAULT.
> > + *
> > + * To make all three error types coexist, we set bit 30 to identify an
> > + * ENCLS fault.  Bit 31 (technically bits N:31) is used to differentiate
> > + * between positive (faults and SGX error codes) and negative (system
> > + * error codes) values.
> > + */
> > +#define ENCLS_FAULT_FLAG 0x40000000
> 
> BIT(30)

This is intentionally open coded so that it can be stringified in asm.
Alternatively, the asm could use the raw value or a different define.  Is
there a third option?

#define __encls_ret_N(rax, inputs...)				\
	({							\
	int ret;						\
	asm volatile(						\
	"1: .byte 0x0f, 0x01, 0xcf;\n\t"			\
	"2:\n"							\
	".section .fixup,\"ax\"\n"				\
	"3: orl $"__stringify(ENCLS_FAULT_FLAG)",%%eax\n"	\  <----
	"   jmp 2b\n"						\
	".previous\n"						\
	_ASM_EXTABLE_FAULT(1b, 3b)				\
	: "=a"(ret)						\
	: "a"(rax), inputs					\
	: "memory", "cc");					\
	ret;							\
	})