Re: [PATCH 4/5] x86: entry_64.S: always allocate complete "struct pt_regs"

[Alexei Starovoitov <ast@plumgrid.com>]

On Fri, Aug 1, 2014 at 7:48 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> 64-bit code was using six stack slots fewer by not saving/restoring
> registers which a callee-preserved according to C ABI,
> and not allocating space for them.
>
> Only when syscall needed a complete "struct pt_regs",
> the complete area was allocated and filled in.
>
> This proved to be a source of significant obfuscation and subtle bugs.
> For example, stub_fork had to pop the return address,
> extend the struct, save registers, and push return address back. Ugly.
> ia32_ptregs_common pops return address and "returns" via jmp insn,
> throwing a wrench into CPU return stack cache.
>
> This patch changes code to always allocate a complete "struct pt_regs".
> The saving of registers is still done lazily.
>
> Macros which manipulate "struct pt_regs" on stack are reworked:
> ALLOC_PTREGS_ON_STACK allocates the structure.
> SAVE_C_REGS saves to it those registers which are clobbered by C code.
> SAVE_EXTRA_REGS saves to it all other registers.
> Corresponding RESTORE_* and REMOVE_PTREGS_FROM_STACK macros reverse it.

Looks like a nice cleanup at the cost of extra 48 byte stack gap for fast path.
I'm guessing the gap shouldn't affect performance, but would be nice
to see performance numbers before/after.