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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Dec 06, 2021 at 05:09:42PM +0100, Jan Beulich wrote:
> 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?

I did that so I could assign from one CPUID policy to another, but
will revert back to using a featureset since it's indeed smaller.

> > +        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?

Will do.

Thanks, Roger.