Re: [PATCH v5 22/26] KVM: VMX: Move LOAD_IA32_PERF_GLOBAL_CTRL errata handling out of setup_vmcs_config()

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 02, 2022, Vitaly Kuznetsov wrote:
> While it seems reasonable to not expose LOAD_IA32_PERF_GLOBAL_CTRL controls
> to L1 hypervisor on buggy CPUs, such change would inevitably break live
> migration from older KVMs where the controls are exposed. Keep the status quo
> for now, L1 hypervisor itself is supposed to take care of the errata.

As noted before, this statement is wrong as it requires guest FMS == host FMS,
but it's irrelevant because KVM can emulate the control unconditionally.  I'll
test and fold in my suggested patch[*] (assuming it works) and reword this part
of the changelog.  Ah, and I'll also need to fold in a patch to actually emulate
the controls without hardware support.

[*] https://lore.kernel.org/all/YtnZmCutdd5tpUmz@google.com

> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 59 +++++++++++++++++++++++++-----------------
>  1 file changed, 35 insertions(+), 24 deletions(-)
> 

...

> @@ -8192,6 +8199,10 @@ static __init int hardware_setup(void)
>  	if (setup_vmcs_config(&vmcs_config, &vmx_capability) < 0)
>  		return -EIO;
>  
> +	if (cpu_has_perf_global_ctrl_bug())
> +		pr_warn_once("kvm: VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL "
> +			     "does not work properly. Using workaround\n");

Any objections to opportunistically tweaking this?

		pr_warn_once("kvm: CPU has VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL erratum,"
			     "using MSR load/store lists for PERF_GLOBAL_CTRL\n");

> +
>  	if (boot_cpu_has(X86_FEATURE_NX))
>  		kvm_enable_efer_bits(EFER_NX);
>  
> -- 
> 2.35.3
>