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Re: [PATCH V2 2/3] target/i386: call cpu_exec_realizefn before x86_cpu_f
From: |
Chen, Zide |
Subject: |
Re: [PATCH V2 2/3] target/i386: call cpu_exec_realizefn before x86_cpu_filter_features |
Date: |
Fri, 31 May 2024 10:13:47 -0700 |
User-agent: |
Mozilla Thunderbird |
On 5/30/2024 11:30 PM, Zhao Liu wrote:
> 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,
Yes, that's why this patches intends to let x86_cpu_filter_features()
filter out the MWAIT bit which is set from the overcommit option.
> and the changes of
> CPUID after x86_cpu_filter_features() should by default be regarded like
> "QEMU knows what it is doing".
Sure, we can add feature bits after x86_cpu_filter_features(), but I
think moving cpu_exec_realizefn() before x86_cpu_filter_features() is
more generic, and actually this is what QEMU did before commit 662175b91ff2.
- Less redundant code. Specifically, no need to call
x86_cpu_get_supported_feature_word() again.
- Potentially there could be other features could be added from the
accel-specific realizefn, kvm_cpu_realizefn() for example. And these
features need to be checked against the host availability.
>
> 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()).
Besides the above reasons, it seems to me expanding env->features in
host-cpu.c is confusing.
> Thanks,
> Zhao
>