Re: [PATCH v5 07/12] libs/guest: introduce helper set cpu topology in cpu policy

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.11.2021 16:33, Roger Pau Monne wrote:
> @@ -458,22 +456,6 @@ int xc_cpuid_apply_policy(xc_interface *xch, uint32_t 
> domid, bool restore,
>           (p = calloc(1, sizeof(*p))) == NULL )
>          goto out;
>  
> -    /* Get the host policy. */
> -    rc = xc_get_cpu_featureset(xch, XEN_SYSCTL_cpu_featureset_host,
> -                               &len, host_featureset);

You go from retrieving the host featureset to ...

> @@ -944,3 +865,98 @@ int xc_cpu_policy_make_compat_4_12(xc_interface *xch, 
> xc_cpu_policy_t *policy,
>      xc_cpu_policy_destroy(host);
>      return rc;
>  }
> +
> +int xc_cpu_policy_legacy_topology(xc_interface *xch, xc_cpu_policy_t *policy,
> +                                  bool hvm)
> +{
> +    if ( !hvm )
> +    {
> +        xc_cpu_policy_t *host;
> +        int rc;
> +
> +        host = xc_cpu_policy_init();
> +        if ( !host )
> +        {
> +            errno = ENOMEM;
> +            return -1;
> +        }
> +
> +        rc = xc_cpu_policy_get_system(xch, XEN_SYSCTL_cpu_policy_host, host);

... retrieving the host policy, which afaict is a larger blob of data.
Is there a particular reason for doing so?

> +        if ( rc )
> +        {
> +            ERROR("Failed to get host policy");
> +            xc_cpu_policy_destroy(host);
> +            return rc;
> +        }
> +
> +
> +        /*
> +         * On hardware without CPUID Faulting, PV guests see real topology.
> +         * As a consequence, they also need to see the host htt/cmp fields.
> +         */
> +        policy->cpuid.basic.htt = host->cpuid.basic.htt;
> +        policy->cpuid.extd.cmp_legacy = host->cpuid.extd.cmp_legacy;
> +    }
> +    else
> +    {
> +        unsigned int i;
> +
> +        /*
> +         * Topology for HVM guests is entirely controlled by Xen.  For now, 
> we
> +         * hardcode APIC_ID = vcpu_id * 2 to give the illusion of no SMT.
> +         */
> +        policy->cpuid.basic.htt = true;
> +        policy->cpuid.extd.cmp_legacy = false;
> +
> +        /*
> +         * Leaf 1 EBX[23:16] is Maximum Logical Processors Per Package.
> +         * Update to reflect vLAPIC_ID = vCPU_ID * 2, but make sure to avoid
> +         * overflow.
> +         */
> +        if ( !policy->cpuid.basic.lppp )
> +            policy->cpuid.basic.lppp = 2;
> +        else if ( !(policy->cpuid.basic.lppp & 0x80) )
> +            policy->cpuid.basic.lppp *= 2;
> +
> +        switch ( policy->cpuid.x86_vendor )
> +        {
> +        case X86_VENDOR_INTEL:
> +            for ( i = 0; (policy->cpuid.cache.subleaf[i].type &&
> +                          i < ARRAY_SIZE(policy->cpuid.cache.raw)); ++i )
> +            {
> +                policy->cpuid.cache.subleaf[i].cores_per_package =
> +                  (policy->cpuid.cache.subleaf[i].cores_per_package << 1) | 
> 1;
> +                policy->cpuid.cache.subleaf[i].threads_per_cache = 0;
> +            }
> +            break;
> +
> +        case X86_VENDOR_AMD:
> +        case X86_VENDOR_HYGON:
> +            /*
> +             * Leaf 0x80000008 ECX[15:12] is ApicIdCoreSize.
> +             * Leaf 0x80000008 ECX[7:0] is NumberOfCores (minus one).
> +             * Update to reflect vLAPIC_ID = vCPU_ID * 2.  But avoid
> +             * - overflow,
> +             * - going out of sync with leaf 1 EBX[23:16],
> +             * - incrementing ApicIdCoreSize when it's zero (which changes 
> the
> +             *   meaning of bits 7:0).
> +             *
> +             * UPDATE: I addition to avoiding overflow, some

Nit: Would you mind switching "I" to "In" at this occasion?

Jan