linux-kernel - Re: ia32_sysenter_target does not preserve EFLAGS

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Message-ID: <CA+55aFwO++rNZM-6UL2A2hRcC_tUAvEd+AeQtSSZXhozn3h-=g@mail.gmail.com>
Date:	Fri, 27 Mar 2015 14:02:46 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: ia32_sysenter_target does not preserve EFLAGS

On Fri, Mar 27, 2015 at 1:53 PM, Brian Gerst <brgerst@...il.com> wrote:
>> <-- IRQ.  Boom
>
> The sti will delay interrupts for one instruction, and that should include NMIs.

Nope. Intel explicitly documents the NMI case only for mov->ss and popss.

> The Intel SDM states for STI:
> "The IF flag and the STI and CLI instructions do not prohibit the
> generation of exceptions and NMI interrupts. NMI
> interrupts (and SMIs) may be blocked for one macroinstruction following an STI."

Note the *may*. For movss and popss the software developer guide
explicitly says that NMI's are also blocked.

For plain sti, it seems to be dependent on microarchitecture.

                     Linus
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