Re: [Xen-devel] [PATCH v2] libxl: disallow PCI device assignment for HVM guest when PoD is enabled

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-01-13 at 11:52 +0000, Wei Liu wrote:
> This replicates a Xend behavior, see ec789523749 ("xend: Dis-allow
> device assignment if PoD is enabled.").
> 
> This change is restricted to HVM guest, as only HVM is relevant in the
> counterpart in Xend. We're late in release cycle so the change should
> only do what's necessary. Probably we can revisit it if we need to do
> the same thing for PV guest in the future.
> 
> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>

Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

Release hat: The risk here is of a false positive detecting whether PoD
would be used and therefore refusing to start a domain. However Wei
directed me earlier on to the code in setup_guest which sets
XENMEMF_populate_on_demand and I believe it is using the same logic.

The benefit of this is that it will stop people starting a domain in an
invalid configuration -- but what is the downside here? Is it an
unhandled IOMMU fault or another host-fatal error? That would make the
argument for taking this patch pretty strong. On the other hand if the
failure were simply to kill this domain, that would be a less serious
issue and I'd be in two minds, mainly due to George not being here to
confirm that the pod_enabled logic is correct (although if he were here
I wouldn't be wrestling with this question at all ;-)).

I'm leaning towards taking this fix, but I'd really like to know what
the current failure case looks like.

Ian.

> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> ---
> v2: fix comment
> ---
>  tools/libxl/libxl_create.c |   20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
> index e03bb55..61437de 100644
> --- a/tools/libxl/libxl_create.c
> +++ b/tools/libxl/libxl_create.c
> @@ -706,6 +706,7 @@ static void initiate_domain_create(libxl__egc *egc,
>      libxl_ctx *ctx = libxl__gc_owner(gc);
>      uint32_t domid;
>      int i, ret;
> +    bool pod_enabled = false;
>  
>      /* convenience aliases */
>      libxl_domain_config *const d_config = dcs->guest_config;
> @@ -714,6 +715,25 @@ static void initiate_domain_create(libxl__egc *egc,
>  
>      domid = 0;
>  
> +    /* If target_memkb is smaller than max_memkb, the subsequent call
> +     * to libxc when building HVM domain will enable PoD mode.
> +     */
> +    pod_enabled = (d_config->c_info.type == LIBXL_DOMAIN_TYPE_HVM) &&
> +        (d_config->b_info.target_memkb < d_config->b_info.max_memkb);
> +
> +    /* We cannot have PoD and PCI device assignment at the same time
> +     * for HVM guest. It was reported that IOMMU cannot work with PoD
> +     * enabled because it needs to populated entire page table for
> +     * guest. To stay on the safe side, we disable PCI device
> +     * assignment when PoD is enabled.
> +     */
> +    if (d_config->c_info.type == LIBXL_DOMAIN_TYPE_HVM &&
> +        d_config->num_pcidevs && pod_enabled) {
> +        ret = ERROR_INVAL;
> +        LOG(ERROR, "PCI device assignment for HVM guest failed due to PoD 
> enabled");
> +        goto error_out;
> +    }
> +
>      ret = libxl__domain_create_info_setdefault(gc, &d_config->c_info);
>      if (ret) goto error_out;
>  



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