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

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