Re: [PATCH 6/6] x86/boot: Expose MSR_ARCH_CAPS data in guest max policies

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 16/05/2023 3:58 pm, Jan Beulich wrote:
> On 15.05.2023 16:42, Andrew Cooper wrote:
>> --- a/xen/arch/x86/cpu-policy.c
>> +++ b/xen/arch/x86/cpu-policy.c
>> @@ -408,6 +408,25 @@ static void __init calculate_host_policy(void)
>>      p->platform_info.cpuid_faulting = cpu_has_cpuid_faulting;
>>  }
>>  
>> +static void __init guest_common_max_feature_adjustments(uint32_t *fs)
>> +{
>> +    if ( boot_cpu_data.x86_vendor == X86_VENDOR_INTEL )
>> +    {
>> +        /*
>> +         * MSR_ARCH_CAPS is just feature data, and we can offer it to guests
>> +         * unconditionally, although limit it to Intel systems as it is 
>> highly
>> +         * uarch-specific.
>> +         *
>> +         * In particular, the RSBA and RRSBA bits mean "you might migrate 
>> to a
>> +         * system where RSB underflow uses alternative predictors (a.k.a
>> +         * Retpoline not safe)", so these need to be visible to a guest in 
>> all
>> +         * cases, even when it's only some other server in the pool which
>> +         * suffers the identified behaviour.
>> +         */
>> +        __set_bit(X86_FEATURE_ARCH_CAPS, fs);
>> +    }
>> +}
> Wouldn't this better be accompanied by marking the bit !a in the public 
> header?

Yes, probably.

~Andrew