Re: [Xen-devel] [PATCH 6/8] tools/libxc: Rework xc_cpuid_apply_policy() to use {get, set}_cpu_policy()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 12/09/2019 10:02, Jan Beulich wrote:
> On 11.09.2019 22:05, Andrew Cooper wrote:
>> The purpose of this change is to stop using xc_cpuid_do_domctl(), and to stop
>> basing decisions on a local CPUID instruction.  This is not a correct or
>> appropriate way to construct policy information for other domains.
>>
>> The overwhelming majority of this logic is redundant with the policy logic in
>> Xen, but has a habit of becoming stale (e.g. c/s 97e4ebdcd76 resulting in
>> AVX512_BF16 not ever actually being offered to guests).
> Well, not offering it to guests was intentional at that point,
> but I guess you validly imply that by adding the A marker to the
> public header it _still_ wouldn't have got exposed?

Ah - I'd forgotten the point about the the lack of A marker, but yes -
my point was that, had I not forgotten during review, that changeset
should have added the 7a1 case to libxc.

I'll see if I can rephrase this slightly to be clearer.

>
>> ---
>>  tools/libxc/xc_cpuid_x86.c      | 798 
>> ++++++++++------------------------------
>>  xen/include/xen/lib/x86/cpuid.h |  11 +-
>>  2 files changed, 197 insertions(+), 612 deletions(-)
> Nice.

I was very pleased with how this turned out.  When I started, I wasn't
expecting to be able to delete this much.

>
>> @@ -1057,3 +449,191 @@ int xc_cpuid_set(
>>  
>>      return rc;
>>  }
>> +
>> +int xc_cpuid_apply_policy(xc_interface *xch, uint32_t domid,
>> +                          const uint32_t *featureset, unsigned int 
>> nr_features)
>> +{
>> +    int rc;
>> +    xc_dominfo_t di;
>> +    unsigned int i, nr_leaves, nr_msrs;
>> +    xen_cpuid_leaf_t *leaves = NULL;
>> +    struct cpuid_policy *p = NULL;
>> +    uint32_t err_leaf = -1, err_subleaf = -1, err_msr = -1;
>> +
>> +    if ( xc_domain_getinfo(xch, domid, 1, &di) != 1 ||
>> +         di.domid != domid )
>> +    {
>> +        ERROR("Failed to obtain d%d info", domid);
>> +        rc = -ESRCH;
>> +        goto out;
>> +    }
>> +
>> +    rc = xc_get_cpu_policy_size(xch, &nr_leaves, &nr_msrs);
>> +    if ( rc )
>> +    {
>> +        PERROR("Failed to obtain policy info size");
>> +        rc = -errno;
>> +        goto out;
>> +    }
>> +
>> +    rc = -ENOMEM;
>> +    if ( (leaves = calloc(nr_leaves, sizeof(*leaves))) == NULL ||
>> +         (p = calloc(1, sizeof(*p))) == NULL )
>> +        goto out;
>> +
>> +    nr_msrs = 0;
>> +    rc = xc_get_domain_cpu_policy(xch, domid, &nr_leaves, leaves,
>> +                                  &nr_msrs, NULL);
>> +    if ( rc )
>> +    {
>> +        PERROR("Failed to obtain d%d's policy", domid);
>> +        rc = -errno;
>> +        goto out;
>> +    }
>> +
>> +    rc = x86_cpuid_copy_from_buffer(p, leaves, nr_leaves,
>> +                                    &err_leaf, &err_subleaf);
>> +    if ( rc )
>> +    {
>> +        ERROR("Failed to deserialise CPUID (err leaf %#x, subleaf %#x) (%d 
>> = %s)",
>> +              err_leaf, err_subleaf, -rc, strerror(-rc));
>> +        goto out;
>> +    }
>> +
>> +    if ( featureset )
>> +    {
>> +        uint32_t disabled_features[FEATURESET_NR_ENTRIES],
>> +            feat[FEATURESET_NR_ENTRIES] = {};
>> +        static const uint32_t deep_features[] = INIT_DEEP_FEATURES;
>> +        unsigned int i, b;
>> +
>> +        /*
>> +         * The user supplied featureset may be shorter or longer than
>> +         * FEATURESET_NR_ENTRIES.  Shorter is fine, and we will zero-extend.
>> +         * Longer is fine, so long as it only padded with zeros.
>> +         */
>> +        unsigned int user_len = min(FEATURESET_NR_ENTRIES + 0u, 
>> nr_features);
>> +
>> +        /* Check for truncated set bits. */
>> +        rc = -EOPNOTSUPP;
>> +        for ( i = user_len; i < nr_features; ++i )
>> +            if ( featureset[i] != 0 )
>> +                goto out;
>> +
>> +        memcpy(feat, featureset, sizeof(*featureset) * user_len);
>> +
>> +        /* Disable deep dependencies of disabled features. */
>> +        for ( i = 0; i < ARRAY_SIZE(disabled_features); ++i )
>> +            disabled_features[i] = ~feat[i] & deep_features[i];
>> +
>> +        for ( b = 0; b < sizeof(disabled_features) * CHAR_BIT; ++b )
>> +        {
>> +            const uint32_t *dfs;
>> +
>> +            if ( !test_bit(b, disabled_features) ||
>> +                 !(dfs = x86_cpuid_lookup_deep_deps(b)) )
>> +                continue;
>> +
>> +            for ( i = 0; i < ARRAY_SIZE(disabled_features); ++i )
>> +            {
>> +                feat[i] &= ~dfs[i];
>> +                disabled_features[i] &= ~dfs[i];
>> +            }
>> +        }
>> +
>> +        cpuid_featureset_to_policy(feat, p);
>> +    }
>> +
>> +    if ( !di.hvm )
>> +    {
>> +        uint32_t host_featureset[FEATURESET_NR_ENTRIES];
>> +        uint32_t len = ARRAY_SIZE(host_featureset);
>> +
>> +        rc = xc_get_cpu_featureset(xch, XEN_SYSCTL_cpu_featureset_host,
>> +                                   &len, host_featureset);
>> +        if ( rc )
>> +        {
>> +            /* Tolerate "buffer too small", as we've got the bits we need. 
>> */
>> +            if ( errno == ENOBUFS )
>> +                rc = 0;
> So this is where I think returning an error (instead of a positive
> number) from the hypercall is latently problematic: There's not
> really any guarantee for ENOBUFS to not result from other than the
> actual hypercall. I guess we have such dependencies elsewhere, so
> having one more here isn't a big deal,

We have it in multiple places.  Adjusting this is going to require some
careful checking of the errno handling.

> but as a precaution against
> using uninitialized data, wouldn't it be prudent for
> host_featureset[] to get zero-initialized up front?

Fair enough.

>
>> +            else
>> +            {
>> +                PERROR("Failed to obtain host featureset");
>> +                rc = -errno;
>> +                goto out;
>> +            }
>> +        }
>> +
>> +        /*
>> +         * On hardware without CPUID Faulting, PV guests see real topology.
>> +         * As a consequence, they also need to see the host htt/cmp fields.
>> +         */
>> +        p->basic.htt       = test_bit(X86_FEATURE_HTT, host_featureset);
>> +        p->extd.cmp_legacy = test_bit(X86_FEATURE_CMP_LEGACY, 
>> host_featureset);
>> +    }
>> +    else
>> +    {
>> +        /*
>> +         * 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.
>> +         */
>> +        p->basic.htt = true;
>> +        p->extd.cmp_legacy = false;
>> +
>> +        p->basic.lppp *= 2;
>> +
>> +        switch ( p->x86_vendor )
>> +        {
>> +        case X86_VENDOR_INTEL:
>> +            for ( i = 0; (p->cache.subleaf[i].type &&
>> +                          i < ARRAY_SIZE(p->cache.raw)); ++i )
>> +            {
>> +                p->cache.subleaf[i].cores_per_package =
>> +                    (p->cache.subleaf[i].cores_per_package << 1) | 1;
>> +                p->cache.subleaf[i].threads_per_cache = 0;
>> +            }
>> +            break;
> The original code masked EDX by 0x3ff. I don't see how this is reflected
> here, and the description also doesn't indicate the change is on purpose.

This falls into the "The overwhelming majority of this logic is
redundant with the policy logic in Xen" statement.

I've got no idea why 0x3ff was used before - only the bottom 3 bits are
defined, and I'm pretty sure that wasn't wasn't the case when this code
first appeared (c/s 5f14a87ceb in 2008).

>
>> +        case X86_VENDOR_AMD:
>> +        case X86_VENDOR_HYGON:
>> +            p->extd.nc = (p->extd.nc << 1) | 1;
> This actually fixes a latent "spill into bit 8" issue of the original
> code.

Oh.  So it does.

~Andrew

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