Re: [PATCH v3 4/5] libxl: update vcpus bitmap in retrieved guest config

[Ian Jackson <ian.jackson@eu.citrix.com>]

Wei Liu writes ("[PATCH v3 4/5] libxl: update vcpus bitmap in retrieved guest config"):
> ... because the available vcpu bitmap can change during domain life time
> due to cpu hotplug and unplug.
> 
> For QEMU upstream, we interrogate QEMU for the number of vcpus. For
> others, we look directly into xenstore for information.
...
> +static int libxl__update_avail_vcpus_qmp(libxl__gc *gc, uint32_t domid,
> +                                         unsigned int max_vcpus,
> +                                         libxl_bitmap *map)
> +{
> +    int rc;
> +
> +    /* For QEMU upstream we always need to return the number
> +     * of cpus present to QEMU whether they are online or not;
> +     * otherwise QEMU won't accept the saved state.

This comment is confusing to me.  Perhaps `return' should read
`provide' ?  But the connection between the body of this function and
the comment is still obscure.

> +static int libxl__update_avail_vcpus_xenstore(libxl__gc *gc, uint32_t domid,
> +                                              unsigned int max_vcpus,
> +                                              libxl_bitmap *map)
> +{
> +    int rc;
> +    unsigned int i;
> +    const char *dompath;
...
> +    for (i = 0; i < max_vcpus; i++) {
> +        const char *path = GCSPRINTF("%s/cpu/%u/availability", dompath, i);
> +        const char *content = libxl__xs_read(gc, XBT_NULL, path);
> +        if (!strcmp(content, "online"))
> +            libxl_bitmap_set(map, i);

Firstly, this should be libxl__xs_read_checked.  Secondly if "content"
is NULL, this will crash.

> @@ -7294,6 +7341,55 @@ int libxl_retrieve_domain_configuration(libxl_ctx *ctx, uint32_t domid,
>          libxl_dominfo_dispose(&info);
>      }
>  
> +    /* VCPUs */
> +    {
> +        libxl_bitmap *map = &d_config->b_info.avail_vcpus;
> +        unsigned int max_vcpus = d_config->b_info.max_vcpus;
> +        libxl_device_model_version version;
> +
> +        libxl_bitmap_dispose(map);
> +        libxl_bitmap_init(map);
> +        libxl_bitmap_alloc(CTX, map, max_vcpus);
> +        libxl_bitmap_set_none(map);

I see that this function already trashes the domain config when it
fails (leaving it up to the caller to free the partial config) but
that this is not documented :-/.

> +        /* If the user did not explicitly specify a device model
> +         * version, we need to use the same logic used during domain
> +         * creation to derive the device model version.
> +         */
> +        version = d_config->b_info.device_model_version;
> +        if (version == LIBXL_DEVICE_MODEL_VERSION_UNKNOWN)
> +            version = libxl__get_device_model_version(gc, &d_config->b_info);

I think this is rather unfortunate.  Won't changing the default device
model while the domain is running will cause this function to give
wrong answers ?

Thanks,
Ian.

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