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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Thomas Gleixner' <tglx@linutronix.de>,
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        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
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        Juergen Gross <jgross@suse.com>,
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        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
Subject: RE: [patch 00/38] x86/retbleed: Call depth tracking mitigation
Thread-Topic: [patch 00/38] x86/retbleed: Call depth tracking mitigation
Thread-Index: AQHYmWozi03gNC/QIkePUsvbFv4MOq2CTJVggABNmoCAADzr8A==
Date:   Sun, 17 Jul 2022 17:56:28 +0000
Message-ID: <24673619e9e2411bb1d5f287aab2aa87@AcuMS.aculab.com>
References: <20220716230344.239749011@linutronix.de>
 <f9fd86acac4f49bc8f90b403978e9df3@AcuMS.aculab.com> <8735ezye00.ffs@tglx>
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From: Thomas Gleixner
> Sent: 17 July 2022 16:07
> 
> On Sun, Jul 17 2022 at 09:45, David Laight wrote:
> > From: Thomas Gleixner
> >>
> >>  3) Utilize the retbleed return thunk mechanism by making the jump
> >>     target run-time configurable. Add the accounting counterpart and
> >>     stuff RSB on underflow in that alternate implementation.
> >
> > What happens to indirect calls?
> > The above would imply that they miss the function entry thunk, but
> > get the return one.
> > Won't this lead to mis-counting of the RSB?
> 
> That's accounted in the indirect call thunk. This mitigation requires
> retpolines enabled.

Thanks, that wasn't in the summary.

> > I also thought that retpolines would trash the return stack?
> 
> No. They prevent that the CPU misspeculates an indirect call due to a
> mistrained BTB.
> 
> > Using a single retpoline thunk would pretty much ensure that
> > they are never correctly predicted from the BTB, but it only
> > gives a single BTB entry that needs 'setting up' to get mis-
> > prediction.
> 
>   BTB != RSB

I was thinking about what happens after the RSB has underflowed.
Which is when (I presume) the BTB based speculation happens.

> The intra function call in the retpoline is of course adding a RSB entry
> which points to the speculation trap, but that gets popped immediately
> after that by the return which goes to the called function.

I'm remembering the 'active' instructions in a retpoline being 'push; ret'.
Which is an RSB imbalance.

...
> > I'm also sure I managed to infer from a document of instruction
> > timings and architectures that some x86 cpu actually used the BTB
> > for normal conditional jumps?
> 
> That's relevant to the problem at hand in which way?

The next problem :-)

	David

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