On Mon, Mar 30, 2020 at 11:08:07AM -0700, Sean Christopherson wrote: > Make __vdso_sgx_enter_enclave() callable from C by preserving %rbx > and taking @leaf in %rcx instead of %rax. Being able to invoke the vDSO > from C reduces the overhead of runtimes that are tightly coupled with > their enclaves, e.g. that can rely on the enclave to save and restore > non-volatile registers, as the runtime doesn't need an assembly wrapper > to preserve non-volatile registers and/or shuffle stack arguments. > > Note, both %rcx and %rbx are consumed by EENTER/ERESUME, i.e. consuming > them doesn't violate the primary tenet of __vdso_sgx_enter_enclave() > that "thou shalt not restrict how information is exchanged between an > enclave and its host process". > > Suggested-by: Nathaniel McCallum <npmccallum@xxxxxxxxxx> > Cc: Cedric Xing <cedric.xing@xxxxxxxxx> > Cc: Jethro Beekman <jethro@xxxxxxxxxxxx> > Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> > Cc: linux-sgx@xxxxxxxxxxxxxxx > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> > --- > arch/x86/entry/vdso/vsgx_enter_enclave.S | 30 ++++++++++++++---------- > 1 file changed, 18 insertions(+), 12 deletions(-) > > diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S > index 34cee2b0ef09..c56064fb36bc 100644 > --- a/arch/x86/entry/vdso/vsgx_enter_enclave.S > +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S > @@ -17,22 +17,22 @@ > > /** > * __vdso_sgx_enter_enclave() - Enter an SGX enclave > + * @rdi: Pass-through value for RDI > + * @rsi: Pass-through value for RSI > + * @rdx: Pass-through value for RDX > * @leaf: ENCLU leaf, must be EENTER or ERESUME > + * @r8: Pass-through value for R8 > + * @r9: Pass-through value for R9 > * @tcs: TCS, must be non-NULL > * @e: Optional struct sgx_enclave_exception instance > * @handler: Optional enclave exit handler > * > - * **Important!** __vdso_sgx_enter_enclave() is **NOT** compliant with the > - * x86-64 ABI, i.e. cannot be called from standard C code. > - * > - * Input ABI: > - * @leaf %eax > - * @tcs 8(%rsp) > - * @e 0x10(%rsp) > - * @handler 0x18(%rsp) > - * > - * Output ABI: > - * @ret %eax > + * **Important!** __vdso_sgx_enter_enclave() does not ensure full compliance I'd simply put **NOTE** here instead of **Important!** as it is more common. > + * with the x86-64 ABI, e.g. doesn't explicitly clear EFLAGS.DF after EEXIT. > + * Except for non-volatile general purpose registers, preserving/setting state > + * in accordance with the x86-64 ABI is the responsibility of the enclave and > + * its runtime, i.e. __vdso_sgx_enter_enclave() cannot be called from C code > + * without careful consideration by both the enclave and its runtime. Instead "e.g. doesn't explcitly clear EFLAGS.DF after EEXIT" (which is somewhat confusing statement) paragraph should be replaced with a simple enumerated list of differences. Something might be left out but that's cool. Just do your best and it can refined over time to be more exact. > * > * All general purpose registers except RAX, RBX and RCX are passed as-is to > * the enclave. RAX, RBX and RCX are consumed by EENTER and ERESUME and are > @@ -71,7 +71,9 @@ > */ > #ifdef SGX_KERNEL_DOC > /* C-style function prototype to coerce kernel-doc into parsing the comment. */ > -int __vdso_sgx_enter_enclave(int leaf, void *tcs, > +int __vdso_sgx_enter_enclave(unsigned long rdi, unsigned long rsi, > + unsigned long rdx, unsigned int leaf, > + unsigned long r8, unsigned long r9, void *tcs, > struct sgx_enclave_exception *e, > sgx_enclave_exit_handler_t handler); > #endif > @@ -83,7 +85,10 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave) > .cfi_rel_offset %rbp, 0 > mov %rsp, %rbp > .cfi_def_cfa_register %rbp > + push %rbx > + .cfi_rel_offset %rbx, -8 > > + mov %ecx, %eax > .Lenter_enclave: > /* EENTER <= leaf <= ERESUME */ > cmp $EENTER, %eax > @@ -109,6 +114,7 @@ SYM_FUNC_START(__vdso_sgx_enter_enclave) > jne .Linvoke_userspace_handler > > .Lout: > + pop %rbx > leave > .cfi_def_cfa %rsp, 8 > ret > -- > 2.24.1 >
