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Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX
 enclave call
To:     Nathaniel McCallum <npmccallum@redhat.com>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
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From:   Jethro Beekman <jethro@fortanix.com>
Message-ID: <db12016d-8719-daa0-4742-7d7c43dd464d@fortanix.com>
Date:   Mon, 16 Mar 2020 14:59:28 +0100
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On 2020-03-16 14:57, Nathaniel McCallum wrote:
> On Mon, Mar 16, 2020 at 9:32 AM Jethro Beekman <jethro@fortanix.com> wr=
ote:
>>
>> On 2020-03-15 18:53, Nathaniel McCallum wrote:
>>> On Sat, Mar 14, 2020 at 9:25 PM Jarkko Sakkinen
>>> <jarkko.sakkinen@linux.intel.com> wrote:
>>>>
>>>> On Wed, Mar 11, 2020 at 01:30:07PM -0400, Nathaniel McCallum wrote:
>>>>> Currently, the selftest has a wrapper around
>>>>> __vdso_sgx_enter_enclave() which preserves all x86-64 ABI callee-sa=
ved
>>>>> registers (CSRs), though it uses none of them. Then it calls this
>>>>> function which uses %rbx but preserves none of the CSRs. Then it ju=
mps
>>>>> into an enclave which zeroes all these registers before returning.
>>>>> Thus:
>>>>>
>>>>>   1. wrapper saves all CSRs
>>>>>   2. wrapper repositions stack arguments
>>>>>   3. __vdso_sgx_enter_enclave() modifies, but does not save %rbx
>>>>>   4. selftest zeros all CSRs
>>>>>   5. wrapper loads all CSRs
>>>>>
>>>>> I'd like to propose instead that the enclave be responsible for sav=
ing
>>>>> and restoring CSRs. So instead of the above we have:
>>>>>   1. __vdso_sgx_enter_enclave() saves %rbx
>>>>>   2. enclave saves CSRs
>>>>>   3. enclave loads CSRs
>>>>>   4. __vdso_sgx_enter_enclave() loads %rbx
>>>>>
>>>>> I know that lots of other stuff happens during enclave transitions,=

>>>>> but at the very least we could reduce the number of instructions
>>>>> through this critical path.
>>>>
>>>> What Jethro said and also that it is a good general principle to cut=

>>>> down the semantics of any vdso as minimal as possible.
>>>>
>>>> I.e. even if saving RBX would make somehow sense it *can* be left
>>>> out without loss in terms of what can be done with the vDSO.
>>>
>>> Please read the rest of the thread. Sean and I have hammered out some=

>>> sensible and effective changes.
>>
>> I'm not sure they're sensible? By departing from the ENCLU calling con=
vention, both the VDSO
>> and the wrapper become more complicated.
>=20
> For the vDSO, only marginally. I'm counting +4,-2 instructions in my
> suggestions. For the wrapper, things become significantly simpler.
>=20
>> The wrapper because now it needs to implement all
>> kinds of logic for different behavior depending on whether the VDSO is=
 or isn't available.
>=20
> When isn't the vDSO available?

When you're not on Linux. Or when you're on an old kernel.

--
Jethro Beekman | Fortanix


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