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    SubjectRe: [PATCH v35 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call
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    On 2020-07-07 05:37, Jarkko Sakkinen wrote:
    > From: Sean Christopherson <sean.j.christopherson@intel.com>
    >
    > An SGX runtime must be aware of the exceptions, which happen inside an
    > enclave. Introduce a vDSO call that wraps EENTER/ERESUME cycle and returns
    > the CPU exception back to the caller exactly when it happens.
    >
    > Kernel fixups the exception information to RDI, RSI and RDX. The SGX call
    > vDSO handler fills this information to the user provided buffer or
    > alternatively trigger user provided callback at the time of the exception.
    >
    > The calling convention is custom and does not follow System V x86-64 ABI.
    >
    > Suggested-by: Andy Lutomirski <luto@amacapital.net>
    > Acked-by: Jethro Beekman <jethro@fortanix.com>
    > Tested-by: Jethro Beekman <jethro@fortanix.com>
    > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
    > Co-developed-by: Cedric Xing <cedric.xing@intel.com>
    > Signed-off-by: Cedric Xing <cedric.xing@intel.com>
    > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
    > ---
    > arch/x86/entry/vdso/Makefile | 2 +
    > arch/x86/entry/vdso/vdso.lds.S | 1 +
    > arch/x86/entry/vdso/vsgx_enter_enclave.S | 131 +++++++++++++++++++++++
    > arch/x86/include/asm/enclu.h | 8 ++
    > arch/x86/include/uapi/asm/sgx.h | 98 +++++++++++++++++
    > 5 files changed, 240 insertions(+)
    > create mode 100644 arch/x86/entry/vdso/vsgx_enter_enclave.S
    > create mode 100644 arch/x86/include/asm/enclu.h
    >
    > diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
    > index ebe82b7aecda..f71ad5ebd0c4 100644
    > --- a/arch/x86/entry/vdso/Makefile
    > +++ b/arch/x86/entry/vdso/Makefile
    > @@ -29,6 +29,7 @@ VDSO32-$(CONFIG_IA32_EMULATION) := y
    > vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o
    > vobjs32-y := vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o
    > vobjs32-y += vdso32/vclock_gettime.o
    > +vobjs-$(VDSO64-y) += vsgx_enter_enclave.o
    >
    > # files to link into kernel
    > obj-y += vma.o extable.o
    > @@ -100,6 +101,7 @@ $(vobjs): KBUILD_CFLAGS := $(filter-out $(GCC_PLUGINS_CFLAGS) $(RETPOLINE_CFLAGS
    > CFLAGS_REMOVE_vclock_gettime.o = -pg
    > CFLAGS_REMOVE_vdso32/vclock_gettime.o = -pg
    > CFLAGS_REMOVE_vgetcpu.o = -pg
    > +CFLAGS_REMOVE_vsgx_enter_enclave.o = -pg
    >
    > #
    > # X32 processes use x32 vDSO to access 64bit kernel data.
    > diff --git a/arch/x86/entry/vdso/vdso.lds.S b/arch/x86/entry/vdso/vdso.lds.S
    > index 36b644e16272..4bf48462fca7 100644
    > --- a/arch/x86/entry/vdso/vdso.lds.S
    > +++ b/arch/x86/entry/vdso/vdso.lds.S
    > @@ -27,6 +27,7 @@ VERSION {
    > __vdso_time;
    > clock_getres;
    > __vdso_clock_getres;
    > + __vdso_sgx_enter_enclave;
    > local: *;
    > };
    > }
    > diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
    > new file mode 100644
    > index 000000000000..be7e467e1efb
    > --- /dev/null
    > +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
    > @@ -0,0 +1,131 @@
    > +/* SPDX-License-Identifier: GPL-2.0 */
    > +
    > +#include <linux/linkage.h>
    > +#include <asm/export.h>
    > +#include <asm/errno.h>
    > +#include <asm/enclu.h>
    > +
    > +#include "extable.h"
    > +
    > +#define EX_LEAF 0*8
    > +#define EX_TRAPNR 0*8+4
    > +#define EX_ERROR_CODE 0*8+6
    > +#define EX_ADDRESS 1*8
    > +
    > +.code64
    > +.section .text, "ax"
    > +
    > +SYM_FUNC_START(__vdso_sgx_enter_enclave)
    > + /* Prolog */
    > + .cfi_startproc
    > + push %rbp
    > + .cfi_adjust_cfa_offset 8
    > + .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
    > + jb .Linvalid_leaf
    > + cmp $ERESUME, %eax
    > + ja .Linvalid_leaf
    > +
    > + /* Load TCS and AEP */
    > + mov 0x10(%rbp), %rbx
    > + lea .Lasync_exit_pointer(%rip), %rcx
    > +
    > + /* Single ENCLU serving as both EENTER and AEP (ERESUME) */
    > +.Lasync_exit_pointer:
    > +.Lenclu_eenter_eresume:
    > + enclu

    After thinking about this some more, I'd like to come back to this setup. Prior discussion at https://lkml.org/lkml/2018/11/2/597 . I hope I'm not derailing the discussion so much as to delay the patch set :(

    I previously mentioned “Userspace may want fine-grained control over enclave scheduling” as a reason userspace may want to specify a different AEP, but gave a bad example. Here's a better example: If I'm running my enclave in an M:N threading model (where M user threads run N TCSs, with N > M), an AEX is a good oppurtunity to switch contexts. Yes, I could implement this with alarm() or so, but that adds overhead while missing out on a lot of opportunities for context switching.

    --
    Jethro Beekman | Fortanix

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