On Tue, Apr 22, 2014 at 10:11:27AM -0700, H. Peter Anvin wrote:
> The fastpath interference is:
>
> 1. Testing for an LDT SS selector before IRET. This is actually easier
> than on 32 bits, because on 64 bits the SS:RSP on the stack is always valid.
>
> 2. Testing for an RSP inside the espfix region on exception entry, so we
> can switch back the stack. This has to be done very early in the
> exception entry since the espfix stack is so small. If NMI and #MC
> didn't use IST it wouldn't work at all (but neither would SYSCALL).
>
> #2 currently saves/restores %rdi, which is also saved further down.
> This is obviously wasteful.
Btw, can we runtime-patch the fastpath interference chunk the moment we
see a 16-bit segment? I.e., connect it to the write_idt() path, i.e. in
the hunk you've removed in there and enable the espfix checks there the
moment we load a 16-bit segment.
I know, I know, this is not so important right now but let me put it out
there just the same.
--
Regards/Gruss,
Boris.
Sent from a fat crate under my desk. Formatting is fine.
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