Re: [PATCH v4] KVM: VMX: do not disable interception for MSR_IA32_SPEC_CTRL on eIBRS

[Josh Poimboeuf]

On Wed, May 31, 2023 at 10:41:28AM -0400, Jon Kohler wrote:
> Avoid expensive rdmsr on every VM Exit for MSR_IA32_SPEC_CTRL on
> eIBRS enabled systems iff the guest only sets IA32_SPEC_CTRL[0] (IBRS)
> and not [1] (STIBP) or [2] (SSBD) by not disabling interception in
> the MSR bitmap. Note: this logic is only for eIBRS, as Intel's guidance
> has long been that eIBRS only needs to be set once, so most guests with
> eIBRS awareness should behave nicely. We would not want to accidentally
> regress misbehaving guests on pre-eIBRS systems, who might be spamming
> IBRS MSR without the hypervisor being able to see it today.
> 
> eIBRS enabled guests using just IBRS will only write SPEC_CTRL MSR
> once or twice per vCPU on boot, so it is far better to take those
> VM exits on boot than having to read and save this msr on every
> single VM exit forever. This outcome was suggested on Andrea's commit
> 2f46993d83ff ("x86: change default to spec_store_bypass_disable=prctl spectre_v2_user=prctl")
> however, since interception is still unilaterally disabled, the rdmsr
> tax is still there even after that commit.
> 
> This is a significant win for eIBRS enabled systems as this rdmsr
> accounts for roughly ~50% of time for vmx_vcpu_run() as observed
> by perf top disassembly, and is in the critical path for all
> VM-Exits, including fastpath exits.
> 
> Opportunistically update comments for both MSR_IA32_SPEC_CTRL and
> MSR_IA32_PRED_CMD to make it clear how L1 vs L2 handling works.
> 
> Fixes: 2f46993d83ff ("x86: change default to spec_store_bypass_disable=prctl spectre_v2_user=prctl")
> Signed-off-by: Jon Kohler <jon@xxxxxxxxxxx>

Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

-- 
Josh