Messages in this thread | | | Date | Fri, 9 Apr 2021 11:45:54 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 0/5] Introduce support for PSF mitigation |
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On Thu, Apr 08, 2021 at 09:56:47AM -0500, Saripalli, RK wrote: > Josh, thank you for taking the time to review the patches. > > On 4/7/2021 5:39 PM, Josh Poimboeuf wrote: > > On Tue, Apr 06, 2021 at 10:49:59AM -0500, Ramakrishna Saripalli wrote: > >> Because PSF speculation is limited to the current program context, > >> the impact of bad PSF speculation is very similar to that of > >> Speculative Store Bypass (Spectre v4) > >> > >> Predictive Store Forwarding controls: > >> There are two hardware control bits which influence the PSF feature: > >> - MSR 48h bit 2 – Speculative Store Bypass (SSBD) > >> - MSR 48h bit 7 – Predictive Store Forwarding Disable (PSFD) > >> > >> The PSF feature is disabled if either of these bits are set. These bits > >> are controllable on a per-thread basis in an SMT system. By default, both > >> SSBD and PSFD are 0 meaning that the speculation features are enabled. > >> > >> While the SSBD bit disables PSF and speculative store bypass, PSFD only > >> disables PSF. > >> > >> PSFD may be desirable for software which is concerned with the > >> speculative behavior of PSF but desires a smaller performance impact than > >> setting SSBD. > > > > Hi Ramakrishna, > > > > Is there a realistic scenario where an application would want to disable > > PSF, but not disable SSB? > > It is possible most applications have been reviewed and scrubbed for > SSB-type attacks but PSF-type issues may not have been looked at yet.
It's "possible", but is it realistic? As far as I know, SSB is impractical to scrub an application for.
Do we know of any real-world cases where this option is needed?
-- Josh
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