Re: [PATCH V2 2/3] target/i386: call cpu_exec_realizefn before x86_cpu_filter_features

[Zhao Liu <zhao1.liu@intel.com>]

Hi Zide,

On Fri, May 24, 2024 at 01:00:16PM -0700, Zide Chen wrote:
> Date: Fri, 24 May 2024 13:00:16 -0700
> From: Zide Chen <zide.chen@intel.com>
> Subject: [PATCH V2 2/3] target/i386: call cpu_exec_realizefn before
>  x86_cpu_filter_features
> X-Mailer: git-send-email 2.34.1
> 
> cpu_exec_realizefn which calls the accel-specific realizefn may expand
> features.  e.g., some accel-specific options may require extra features
> to be enabled, and it's appropriate to expand these features in accel-
> specific realizefn.
> 
> One such example is the cpu-pm option, which may add CPUID_EXT_MONITOR.
> 
> Thus, call cpu_exec_realizefn before x86_cpu_filter_features to ensure
> that it won't expose features not supported by the host.
> 
> Fixes: 662175b91ff2 ("i386: reorder call to cpu_exec_realizefn")
> Suggested-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Signed-off-by: Zide Chen <zide.chen@intel.com>
> ---
>  target/i386/cpu.c         | 24 ++++++++++++------------
>  target/i386/kvm/kvm-cpu.c |  1 -
>  2 files changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index bc2dceb647fa..a1c1c785bd2f 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -7604,6 +7604,18 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
>          }
>      }
>  
> +    /*
> +     * note: the call to the framework needs to happen after feature expansion,
> +     * but before the checks/modifications to ucode_rev, mwait, phys_bits.
> +     * These may be set by the accel-specific code,
> +     * and the results are subsequently checked / assumed in this function.
> +     */
> +    cpu_exec_realizefn(cs, &local_err);
> +    if (local_err != NULL) {
> +        error_propagate(errp, local_err);
> +        return;
> +    }
> +
>      x86_cpu_filter_features(cpu, cpu->check_cpuid || cpu->enforce_cpuid);

For your case, which sets cpu-pm=on via overcommit, then
x86_cpu_filter_features() will complain that mwait is not supported.

Such warning is not necessary, because the purpose of overcommit (from
code) is only to support mwait when possible, not to commit to support
mwait in Guest.

Additionally, I understand x86_cpu_filter_features() is primarily
intended to filter features configured by the user, and the changes of
CPUID after x86_cpu_filter_features() should by default be regarded like
"QEMU knows what it is doing".

I feel adding a check for the CPUID mwait bit in host_cpu_realizefn()
is enough, after all, this bit should be present if host supports mwait
and enable_cpu_pm (in kvm_arch_get_supported_cpuid()).

Thanks,
Zhao