Re: [Xen-devel] [PATCH v4 08/24] x86: refactor psr: set value: implement framework.

["Jan Beulich" <JBeulich@xxxxxxxx>]

> To make the set value flow be general and can support multiple features
> at same time, it includes below steps:
> 1. Get COS ID of current domain using.
> 2. Assemble a value array to store all features current value
>    in it and replace the current value of the feature which is
>    being set to the new input value.
> 3. Find if there is already a COS ID on which all features'
>    values are same as the array. Then, we can reuse this COS
>    ID.
> 4. If fail to find, we need allocate a new COS ID. Only COS ID which ref
>    is 0 or 1 can be allocated.

Using "allocate" here in conjunction with ref count being 1 is a little
misleading here - allocation would mean a fresh ID, whereas in the
case of ref == 1 you mean to re-use the current one.

> --- a/xen/arch/x86/psr.c
> +++ b/xen/arch/x86/psr.c
> @@ -513,18 +513,197 @@ int psr_get_val(struct domain *d, unsigned int socket,
>      return -ENOENT;
>  }
>  
> -int psr_set_l3_cbm(struct domain *d, unsigned int socket,
> -                   uint64_t cbm, enum cbm_type type)
> +/* Set value functions */
> +static unsigned int get_cos_num(const struct psr_socket_info *info)
>  {
>      return 0;
>  }
>  
> +static int get_old_set_new(uint64_t *val,
> +                           uint32_t array_len,
> +                           const struct psr_socket_info *info,
> +                           unsigned int old_cos,
> +                           enum cbm_type type,
> +                           uint64_t m)
> +{
> +    return 0;
> +}
> +
> +static int find_cos(const uint64_t *val, uint32_t array_len,
> +                    enum cbm_type type,
> +                    const struct psr_socket_info *info)
> +{
> +    return 0;
> +}
> +
> +static int alloc_new_cos(const struct psr_socket_info *info,
> +                         const uint64_t *val, uint32_t array_len,
> +                         unsigned int old_cos,
> +                         enum cbm_type type)
> +{
> +    return 0;
> +}
> +
> +static int write_psr_msr(unsigned int socket, unsigned int cos,
> +                         const uint64_t *val)
> +{
> +    return 0;
> +}

I think all of the above functions should return an error as long as
they're stubbed out, yet I don't think 0 means error in all cases (in
particular a return value of plain int suggests 0 to mean success).

> +int psr_set_val(struct domain *d, unsigned int socket,
> +                uint64_t val, enum cbm_type type)
> +{
> +    unsigned int old_cos;
> +    int cos, ret;
> +    unsigned int *ref;
> +    uint64_t *val_array;
> +    struct psr_socket_info *info = get_socket_info(socket);
> +    uint32_t array_len;
> +
> +    if ( IS_ERR(info) )
> +        return PTR_ERR(info);
> +
> +    /*
> +     * Step 0:
> +     * old_cos means the COS ID current domain is using. By default, it is 0.
> +     *
> +     * For every COS ID, there is a reference count to record how many 
> domains
> +     * are using the COS register corresponding to this COS ID.
> +     * - If ref[old_cos] is 0, that means this COS is not used by any domain.
> +     * - If ref[old_cos] is 1, that means this COS is only used by current
> +     *   domain.
> +     * - If ref[old_cos] is more than 1, that mean multiple domains are using
> +     *   this COS.
> +     */
> +    old_cos = d->arch.psr_cos_ids[socket];
> +    if ( old_cos > MAX_COS_REG_CNT )
> +        return -EOVERFLOW;
> +
> +    ref = info->cos_ref;
> +
> +    /*
> +     * Step 1:
> +     * Assemle a value array to store all featues cos_reg_val[old_cos].
> +     * And, set the input val into array according to the feature's
> +     * position in array.
> +     */
> +    array_len = get_cos_num((const struct psr_socket_info *)info);

What is this cast doing here? (There are more of this kind below.)

> +    val_array = xzalloc_array(uint64_t, array_len);
> +    if ( !val_array )
> +        return -ENOMEM;
> +
> +    if ( (ret = get_old_set_new(val_array, array_len,
> +                                (const struct psr_socket_info *)info,
> +                                old_cos, type, val)) != 0 )

Just like for earlier versions I continue to be unconvinced that
the get-current-settings and the replace-target-value should be
a single operation. In particular I'd expect the function to be able
to store the target value, as long as the array entries are
ordered in a suitable way.

> +    {
> +        xfree(val_array);
> +        return ret;
> +    }
> +
> +    /*
> +     * Lock here to make sure the ref is not changed during find and
> +     * write process.
> +     */
> +    spin_lock(&info->ref_lock);
> +
> +    /*
> +     * Step 2:
> +     * Try to find if there is already a COS ID on which all features' values
> +     * are same as the array. Then, we can reuse this COS ID.
> +     */
> +    cos = find_cos((const uint64_t *)val_array, array_len, type,
> +                   (const struct psr_socket_info *)info);
> +    if ( cos >= 0 )
> +    {
> +        if ( cos == old_cos )
> +        {
> +            spin_unlock(&info->ref_lock);
> +            xfree(val_array);
> +            return 0;
> +        }
> +    }
> +    else
> +    {
> +        /*
> +         * Step 3:
> +         * If fail to find, we need allocate a new COS ID.
> +         * If multiple domains are using same COS ID, its ref is more
> +         * than 1. That means we cannot free this COS to make current domain
> +         * use it. Because other domains are using the value saved in the 
> COS.
> +         * Unless the ref is changed to 1 (mean only current domain is using
> +         * it), we cannot allocate the COS ID to current domain.
> +         * So, only the COS ID which ref is 1 or 0 can be allocated.
> +         */

I suppose this comment will make more sense when further patches
get applied, as so far there was no ref count check at all anywhere.

> +        cos = alloc_new_cos((const struct psr_socket_info *)info,
> +                            (const uint64_t *)val_array, array_len,
> +                            old_cos, type);
> +        if ( cos < 0 )
> +        {
> +            spin_unlock(&info->ref_lock);
> +            xfree(val_array);
> +            return cos;
> +        }
> +
> +        /*
> +         * Step 4:
> +         * Write all features MSRs according to the COS ID.
> +         */
> +        ret = write_psr_msr(socket, cos, (const uint64_t *)val_array);
> +        if ( ret )
> +        {
> +            spin_unlock(&info->ref_lock);
> +            xfree(val_array);
> +            return ret;
> +        }
> +    }
> +
> +    /*
> +     * Step 5:
> +     * Update ref according to COS ID.
> +     */
> +    ref[cos]++;
> +    ref[old_cos]--;
> +    spin_unlock(&info->ref_lock);
> +
> +    /*
> +     * Step 6:
> +     * Save the COS ID into current domain's psr_cos_ids[] so that we can 
> know
> +     * which COS the domain is using on the socket. One domain can only use
> +     * one COS ID at same time.

To help readers, perhaps this last sentence should be completed with
"... on each socket"?

> +     */
> +    d->arch.psr_cos_ids[socket] = cos;
> +    xfree(val_array);
> +
> +    return 0;
> +}
> +
>  /* Called with domain lock held, no extra lock needed for 'psr_cos_ids' */
>  static void psr_free_cos(struct domain *d)
>  {
> +    unsigned int socket;
> +    unsigned int cos;

These could (and imo should) be joined together.

> +    struct psr_socket_info *info;
> +
>      if( !d->arch.psr_cos_ids )
>          return;
>  
> +    /* Domain is free so its cos_ref should be decreased. */
> +    for( socket = 0; socket < nr_sockets; socket++ )

Missing blank (also in the if() above, not sure by which earlier patch
that got introduced).

> +    {
> +        /* cos 0 is default one which does not need be handled. */
> +        if ( (cos = d->arch.psr_cos_ids[socket]) == 0 )
> +            continue;
> +
> +        /*
> +         * If domain uses other cos ids, all corresponding refs must have 
> been
> +         * increased 1 for this domain. So, we need decrease them.
> +         */
> +        info = socket_info + socket;
> +        spin_lock(&info->ref_lock);
> +        info->cos_ref[cos]--;

While likely also relevant in other places, this one in particular
suggests that you should add ASSERT()s: Before decrements and
after increments the ref count should not be zero.

Also please move the declaration of at least "info" into the most
narrow scope possible.

Jan

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