Re: Dealing with the NMI mess

[Willy Tarreau]

On Fri, Jul 24, 2015 at 07:10:18PM +0200, Willy Tarreau wrote:
> The OS has to set the RSP by itself before doing SYSRET, which opens a
> race between "mov rsp" and "sysret", but if we only take that path once
> we figure we come from NMI (using just IF+RSP), we know that IRQs and
> NMIs are still disabled and cannot strike at this instant. Maybe MCEs
> can, but they would execute within the NMI's stack just as if they were
> triggered inside the NMI as well so I don't see a problem here.

OK too bad I just found the response here in the code :-(

     * SYSRET can't restore RF.  SYSRET can restore TF, but unlike IRET,
     * restoring TF results in a trap from userspace immediately after
     * SYSRET.  This would cause an infinite loop whenever #DB happens
     * with register state that satisfies the opportunistic SYSRET
     * conditions.

Willy

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