Predictive Store Forwarding: AMD Zen3 processors feature a new
technology called Predictive Store Forwarding (PSF).

PSF is a hardware-based micro-architectural optimization designed
to improve the performance of code execution by predicting address
dependencies between loads and stores.

How PSF works:

It is very common for a CPU to execute a load instruction to an address
that was recently written by a store. Modern CPUs implement a technique
known as Store-To-Load-Forwarding (STLF) to improve performance in such
cases. With STLF, data from the store is forwarded directly to the load
without having to wait for it to be written to memory. In a typical CPU,
STLF occurs after the address of both the load and store are calculated
and determined to match.

PSF expands on this by speculating on the relationship between loads and
stores without waiting for the address calculation to complete. With PSF,
the CPU learns over time the relationship between loads and stores. If
STLF typically occurs between a particular store and load, the CPU will
remember this.

In typical code, PSF provides a performance benefit by speculating on
the load result and allowing later instructions to begin execution
sooner than they otherwise would be able to.

The details of security analysis of AMD predictive store forwarding is
documented here.
https://www.amd.com/system/files/documents/security-analysis-predictive-store-forwarding.pdf

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.

Support for PSFD is indicated in CPUID Fn8000_0008 EBX[28].
All processors that support PSF will also support PSFD.

Linux kernel does not have the interface to enable/disable PSFD yet. Plan
here is to expose the PSFD technology to KVM so that the guest kernel can
make use of it if they wish to.
Signed-off-by: Babu Moger <Babu.Moger@amd.com>
Message-Id: <163244601049.30292.5855870305350227855.stgit@bmoger-ubuntu>
[Keep feature private to KVM, as requested by Borislav Petkov. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>