Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

[Jarkko Sakkinen]

On Mon, Oct 05, 2020 at 07:57:05PM -0700, Sean Christopherson wrote:
> On Sat, Oct 03, 2020 at 07:50:56AM +0300, Jarkko Sakkinen wrote:
> > +	__u16 exception_vector;
> > +	__u16 exception_error_code;
> > +	__u64 exception_addr;
> > +	__u8  reserved[24];
> 
> I also think it's a waste of space to bother with multiple reserved fields.
> 24 bytes isn't so much that it guarantees we'll never run into problems in
> the future.  But I care far less about this than I do about exit_reason.

For me the real problem is that there has not been "no brainer" basis
for any size, so a one cache line worth of data is just something that
makes sense, because would neither make much sense to have less.

I'll throw an argument to have it a bit bigger amount of reserved space
for future use.

First, there is always some amount of unknown unknowns when it comes to
run-time structures, given the evolution of microarchitectures. So yes,
some more "state" might be needed in the future.

Secondly, this is a bigger problem for the vDSO than it is for ioctl's
because we can have only one. With ioctl's, in the absolute worst case,
we can have a second version of the same ioctl.

At least 256 bytes would be probably a good number, if we want to
increase it. The reserved space zero validation that I implemented to
this version probably does not add much to the overhead anyway.

I'm not sure why care about one struct field more than making sure that
the run-time structure can stand time.

/Jarkko