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

[Jarkko Sakkinen]

On Wed, Oct 07, 2020 at 12:39:27AM +0300, Jarkko Sakkinen wrote:
> 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.

So what I could do is to grow the reserved area and based on my response
explain this in the changelog message but I need to make sure that I got
the reasoning right behind the size.

/Jarkko