Re: [PATCH] x86/speculation: Mitigate eIBRS PBRSB predictions with WRMSR

From: Jim Mattson
Date: Wed Oct 05 2022 - 18:29:31 EST

On Wed, Oct 5, 2022 at 3:03 PM Suraj Jitindar Singh <surajjs@xxxxxxxxxx> wrote:
> tl;dr: The existing mitigation for eIBRS PBRSB predictions uses an INT3 to
> ensure a call instruction retires before a following unbalanced RET. Replace
> this with a WRMSR serialising instruction which has a lower performance
> penalty.

The INT3 is only on a speculative path and should not impact performance.

> == Background ==
> eIBRS (enhanced indirect branch restricted speculation) is used to prevent
> predictor addresses from one privilege domain from being used for prediction
> in a higher privilege domain.
> == Problem ==
> On processors with eIBRS protections there can be a case where upon VM exit
> a guest address may be used as an RSB prediction for an unbalanced RET if a
> CALL instruction hasn't yet been retired. This is termed PBRSB (Post-Barrier
> Return Stack Buffer).
> A mitigation for this was introduced in:
> (2b1299322016731d56807aa49254a5ea3080b6b3 x86/speculation: Add RSB VM Exit protections)
> This mitigation [1] has a ~1% performance impact on VM exit compared to without
> it [2].
> == Solution ==
> The WRMSR instruction can be used as a speculation barrier and a serialising
> instruction. Use this on the VM exit path instead to ensure that a CALL
> instruction (in this case the call to vmx_spec_ctrl_restore_host) has retired
> before the prediction of a following unbalanced RET.

I don't buy this solution. According to

"Note that a WRMSR instruction (used to set IBRS, for example), could
also serve as a speculation barrier for such a sequence in place of

This says only that you can replace the LFENCE with a WRMSR. It
doesn't say that you can drop the rest of the sequence.

> This mitigation [3] has a negligible performance impact.
> == Testing ==
> Run the outl_to_kernel kvm-unit-tests test 200 times per configuration which
> counts the cycles for an exit to kernel mode.
> [1] With existing mitigation:
> Average: 2026 cycles
> [2] With no mitigation:
> Average: 2008 cycles
> [3] With proposed mitigation:
> Average: 2008 cycles

What testing did you do to see that this is an effective mitigation?
Improved timings are irrelevant if it doesn't work.