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SubjectRe: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call
On Mon, Mar 16, 2020 at 8:27 PM Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Mon, Mar 16, 2020 at 05:18:14PM -0700, Xing, Cedric wrote:
> > On 3/16/2020 4:59 PM, Sean Christopherson wrote:
> > >On Mon, Mar 16, 2020 at 04:50:26PM -0700, Xing, Cedric wrote:
> > >>On 3/16/2020 3:53 PM, Sean Christopherson wrote:
> > >>>On Mon, Mar 16, 2020 at 11:38:24PM +0200, Jarkko Sakkinen wrote:
> > >>>>>My suggestions explicitly maintained robustness, and in fact increased
> > >>>>>it. If you think we've lost capability, please speak with specificity
> > >>>>>rather than in vague generalities. Under my suggestions we can:
> > >>>>>1. call the vDSO from C
> > >>>>>2. pass context to the handler
> > >>>>>3. have additional stack manipulation options in the handler
> > >>>>>
> > >>>>>The cost for this is a net 2 additional instructions. No existing
> > >>>>>capability is lost.
> > >>>>
> > >>>>My vague generality in this case is just that the whole design
> > >>>>approach so far has been to minimize the amount of wrapping to
> > >>>>EENTER.
> > >>>
> > >>>Yes and no. If we wanted to minimize the amount of wrapping around the
> > >>>vDSO's ENCLU then we wouldn't have the exit handler shenanigans in the
> > >>>first place. The whole process has been about balancing the wants of each
> > >>>use case against the overall quality of the API and code.
> > >>>
> > >>The design of this vDSO API was NOT to minimize wrapping, but to allow
> > >>maximal flexibility. More specifically, we strove not to restrict how info
> > >>was exchanged between the enclave and its host process. After all, calling
> > >>convention is compiler specific - i.e. the enclave could be built by a
> > >>different compiler (e.g. MSVC) that doesn't share the same list of CSRs as
> > >>the host process. Therefore, the API has been implemented to pass through
> > >>virtually all registers except those used by EENTER itself. Similarly, all
> > >>registers are passed back from enclave to the caller (or the exit handler)
> > >>except those used by EEXIT. %rbp is an exception because the vDSO API has to
> > >>anchor the stack, using either %rsp or %rbp. We picked %rbp to allow the
> > >>enclave to allocate space on the stack.
> > >
> > >And unless I'm missing something, using %rcx to pass @leaf would still
> > >satisfy the above, correct? Ditto for saving/restoring %rbx.
> > >
> > >I.e. a runtime that's designed to work with enclave's using a different
> > >calling convention wouldn't be able to take advantage of being able to call
> > >the vDSO from C, but neither would it take on any meaningful burden.
> > >
> > Not exactly.
> >
> > If called directly from C code, the caller would expect CSRs to be
> > preserved. Then who should preserve CSRs? It can't be the enclave because it
> > may not follow the same calling convention. Moreover, the enclave may run
> > into an exception, in which case it doesn't have the ability to restore
> > CSRs. So it has to be done by the vDSO API. That means CSRs will be
> > overwritten upon enclave exits, which violates the goal of "passing all
> > registers back to the caller except those used by EEXIT".
>
> IIUC, Nathaniel's use case is to run only enclaves that are compatible
> with Linux's calling convention and to handle enclave exceptions in the
> exit handler.
>
> As I qualified above, there would certainly be runtimes and use cases that
> would find no advantage in passing @leaf via %rcx and preserving %rbx. I'm
> well aware the Intel SDK falls into that bucket. But again, the cost to
> such runtimes is precisely one reg->reg MOV instruction.

It seems to me that some think my proposal represents a shift in
strategic direction. I do not see it that way. I affirm the existing
strategic direction. My proposal only represents a specific
optimization of that strategic direction that benefits certain use
cases without significant cost to all other use cases.

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