Re: [PATCH V3 2/2] target/i386: Advertise MWAIT iff host supports

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 6/4/2024 8:02 AM, Zide Chen wrote:
> host_cpu_realizefn() sets CPUID_EXT_MONITOR without consulting host/KVM
> capabilities. This may cause problems:
> 
> - If MWAIT/MONITOR is not available on the host, advertising this
>    feature to the guest and executing MWAIT/MONITOR from the guest
>    triggers #UD and the guest doesn't boot.  This is because typically
>    #UD takes priority over VM-Exit interception checks and KVM doesn't
>    emulate MONITOR/MWAIT on #UD.
> 
> - If KVM doesn't support KVM_X86_DISABLE_EXITS_MWAIT, MWAIT/MONITOR
>    from the guest are intercepted by KVM, which is not what cpu-pm=on
>    intends to do.
> 
> In these cases, MWAIT/MONITOR should not be exposed to the guest.
> 
> The logic in kvm_arch_get_supported_cpuid() to handle CPUID_EXT_MONITOR
> is correct and sufficient, and we can't set CPUID_EXT_MONITOR after
> x86_cpu_filter_features().
> 
> This was not an issue before commit 662175b91ff ("i386: reorder call to
> cpu_exec_realizefn") because the feature added in the accel-specific
> realizefn could be checked against host availability and filtered out.
> 
> Additionally, it seems not a good idea to handle guest CPUID leaves in
> host_cpu_realizefn(), and this patch merges host_cpu_enable_cpu_pm()
> into kvm_cpu_realizefn().
> 
> Fixes: f5cc5a5c1686 ("i386: split cpu accelerators from cpu.c, using AccelCPUClass")
> Fixes: 662175b91ff2 ("i386: reorder call to cpu_exec_realizefn")
> Signed-off-by: Zide Chen <zide.chen@intel.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

> ---
> 
> V3:
> - don't set CPUID_EXT_MONITOR in kvm_cpu_realizefn().
> - Change title to reflect the main purpose of this patch.
> 
>   target/i386/host-cpu.c    | 12 ------------
>   target/i386/kvm/kvm-cpu.c | 11 +++++++++--
>   2 files changed, 9 insertions(+), 14 deletions(-)
> 
> diff --git a/target/i386/host-cpu.c b/target/i386/host-cpu.c
> index 280e427c017c..8b8bf5afeccf 100644
> --- a/target/i386/host-cpu.c
> +++ b/target/i386/host-cpu.c
> @@ -42,15 +42,6 @@ static uint32_t host_cpu_phys_bits(void)
>       return host_phys_bits;
>   }
>   
> -static void host_cpu_enable_cpu_pm(X86CPU *cpu)
> -{
> -    CPUX86State *env = &cpu->env;
> -
> -    host_cpuid(5, 0, &cpu->mwait.eax, &cpu->mwait.ebx,
> -               &cpu->mwait.ecx, &cpu->mwait.edx);
> -    env->features[FEAT_1_ECX] |= CPUID_EXT_MONITOR;
> -}
> -
>   static uint32_t host_cpu_adjust_phys_bits(X86CPU *cpu)
>   {
>       uint32_t host_phys_bits = host_cpu_phys_bits();
> @@ -83,9 +74,6 @@ bool host_cpu_realizefn(CPUState *cs, Error **errp)
>       X86CPU *cpu = X86_CPU(cs);
>       CPUX86State *env = &cpu->env;
>   
> -    if (cpu->max_features && enable_cpu_pm) {
> -        host_cpu_enable_cpu_pm(cpu);
> -    }
>       if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
>           uint32_t phys_bits = host_cpu_adjust_phys_bits(cpu);
>   
> diff --git a/target/i386/kvm/kvm-cpu.c b/target/i386/kvm/kvm-cpu.c
> index f76972e47e61..148d10ce3711 100644
> --- a/target/i386/kvm/kvm-cpu.c
> +++ b/target/i386/kvm/kvm-cpu.c
> @@ -65,8 +65,15 @@ static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
>        *   cpu_common_realizefn() (via xcc->parent_realize)
>        */
>       if (cpu->max_features) {
> -        if (enable_cpu_pm && kvm_has_waitpkg()) {
> -            env->features[FEAT_7_0_ECX] |= CPUID_7_0_ECX_WAITPKG;
> +        if (enable_cpu_pm) {
> +            if (kvm_has_waitpkg()) {
> +                env->features[FEAT_7_0_ECX] |= CPUID_7_0_ECX_WAITPKG;
> +            }
> +
> +            if (env->features[FEAT_1_ECX] & CPUID_EXT_MONITOR) {
> +                host_cpuid(5, 0, &cpu->mwait.eax, &cpu->mwait.ebx,
> +                           &cpu->mwait.ecx, &cpu->mwait.edx);
> +	    }
>           }
>           if (cpu->ucode_rev == 0) {
>               cpu->ucode_rev =