Re: [PATCH] x86/bhi: avoid hardware mitigation for 'spectre_bhi=vmexit'

[Jim Mattson]

On Thu, Sep 12, 2024 at 10:29 PM Chao Gao <chao.gao@xxxxxxxxx> wrote:
>
> On Thu, Sep 12, 2024 at 09:24:40AM -0700, Pawan Gupta wrote:
> >On Thu, Sep 12, 2024 at 03:44:38PM +0000, Jon Kohler wrote:
> >> > It is only worth implementing the long sequence in VMEXIT_ONLY mode if it is
> >> > significantly better than toggling the MSR.
> >>
> >> Thanks for the pointer! I hadn’t seen that second sequence. I’ll do measurements on
> >> three cases and come back with data from an SPR system.
> >> 1. as-is (wrmsr on entry and exit)
> >> 2. Short sequence (as a baseline)
> >> 3. Long sequence
> >
> >I wonder if virtual SPEC_CTRL feature introduced in below series can
> >provide speedup, as it can replace the MSR toggling with faster VMCS
> >operations:
>
> "virtual SPEC_CTRL" won't provide speedup. the wrmsr on entry/exit is still
> need if guest's (effective) value and host's value are different.

I believe that is the case here. The guest's effective value is 1025.
If the guest knew about BHI_DIS_S, it would actually set it to 1025,
but older guests set it to 1.

The IA32_SPEC_CTRL mask and shadow fields should be perfect for this.

> "virtual SPEC_CTRL" just prevents guests from toggling some bits. It doesn't
> switch the MSR between guest value and host value on entry/exit. so, KVM has
> to do the switching with wrmsr/rdmsr instructions. A new feature, "load
> IA32_SPEC_CTRL" VMX control (refer to Chapter 15 in ISE spec[*]), can help but
> it isn't supported on SPR.
>
> [*]: https://cdrdv2.intel.com/v1/dl/getContent/671368
>
> >
> >  https://lore.kernel.org/kvm/20240410143446.797262-1-chao.gao@xxxxxxxxx/
> >
> >Adding Chao for their opinion.
>