Disabling speculative execution mitigations

[Konstantin Belousov]

On Fri, Dec 06, 2019 at 03:51:04PM +1030, O'Connor, Daniel wrote:
> Hi,
> I am trying to track down a performance drop with the ASPEED xorg video driver between FreeBSD 11 and 12 (I'm not expecting miracles from it but it was basically unusable..)
> 
> I wondered if some of the speculative execution mitigations could be causing the problem so I did some digging and found these..
> 
> vm.pmap.pti="0"        # Disable page table isolation
> hw.ibrs_disable="1"    # Disable Indirect Branch Restricted Speculation
This line enables IBRS.

> hw.mds_disable="0"     # Disable Microarchitectural Data Sampling flush
> hw.vmm.vmx="1"         # Don't flush RSB on vmexit (presumably only affects bhyve etc)
I have no idea what this line should configure.

> hw.lazy_fpu_switch="1" # Lazily flush FPU
> 
> Does anyone know of any others?
Did you read security(7) (on HEAD)?

> 
> I have 2 systems with the same motherboard (Supermicro X11SSH-F), one is older and runs FreeBSD 11 (and had an older BIOS_ and the newer runs FreeBSD 12.
> 
> FWIW on FreeBSD 11 the performance (measured by a subset of x11perf benchmarks) went down 40% after updating to the latest BIOS (2.2a). Unfortunately on FreeBSD 12 rolling back to the original BIOS (2.2) did not improve performance.
> 
> --
> Daniel O'Connor
> "The nice thing about standards is that there
> are so many of them to choose from."
>  -- Andrew Tanenbaum
> 
> 
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