Re: [Xen-devel] [PATCH v12 6/6] introduce a 'passthrough' configuration option to xl.cfg...

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 9/16/19 10:27 AM, Paul Durrant wrote:
> ...and hence the ability to disable IOMMU mappings, and control EPT
> sharing.
> 
> This patch introduces a new 'libxl_passthrough' enumeration into
> libxl_domain_create_info. The value will be set by xl either when it parses
> a new 'passthrough' option in xl.cfg, or implicitly if there is passthrough
> hardware specified for the domain.
> 
> If the value of the passthrough configuration option is 'disabled' then
> the XEN_DOMCTL_CDF_iommu flag will be clear in the xen_domctl_createdomain
> flags, thus allowing the toolstack to control whether the domain gets
> IOMMU mappings or not (where previously they were globally set).
> 
> If the value of the passthrough configuration option is 'sync_pt' then
> a new 'iommu_opts' field in xen_domctl_createdomain will be set with the
> value XEN_DOMCTL_IOMMU_no_sharept. This will override the global default
> set in iommu_hap_pt_share, thus allowing the toolstack to control whether
> EPT sharing is used for the domain.
> 
> If the value of passthrough is 'enabled' then xl will choose an appropriate
> default according to the type of domain and hardware support.

Minor suggestion: I prefer using a word like "auto" when you're letting
the computer decide something.

I'd also...

> diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
> index c99d40307e..154d847fb9 100644
> --- a/docs/man/xl.cfg.5.pod.in
> +++ b/docs/man/xl.cfg.5.pod.in
> @@ -605,6 +605,62 @@ option should only be used with a trusted device tree.
>  Note that the partial device tree should avoid using the phandle 65000
>  which is reserved by the toolstack.
>  
> +=item B<passthrough="STRING">
> +
> +Specify whether IOMMU mappings are enabled for the domain and hence whether
> +it will be enabled for passthrough hardware. Valid values for this option
> +are:
> +
> +=over 4
> +
> +=item B<disabled>
> +
> +IOMMU mappings are disabled for the domain and so hardware may not be
> +passed through.
> +
> +This option is the default if no passthrough hardware is specified in the
> +domain's configuration.
> +
> +=item B<sync_pt>
> +
> +This option means that IOMMU mappings will be synchronized with the
> +domain's P2M table as follows:
> +
> +For a PV domain, all writable pages assigned to the domain are identity
> +mapped by MFN in the IOMMU page table. Thus a device driver running in the
> +domain may program passthrough hardware for DMA using MFN values
> +(i.e. host/machine frame numbers) looked up in its P2M.
> +
> +For an HVM domain, all non-foreign RAM pages present in its P2M will be
> +mapped by GFN in the IOMMU page table. Thus a device driver running in the
> +domain may program passthrough hardware using GFN values (i.e. guest
> +physical frame numbers) without any further translation.
> +
> +This option is not currently available on Arm.
> +
> +=item B<share_pt>
> +
> +This option is unavailable for a PV domain. For an HVM domain, this option
> +means that the IOMMU will be programmed to directly reference the domain's
> +P2M table as its page table. From the point of view of a device driver
> +running in the domain this is functionally equivalent to B<sync_pt> but
> +places less load on the hypervisor and so should generally be selected in
> +preference. However, the availability of this option is hardware specific.
> +If B<xl info> reports B<virt_caps> containing B<iommu_hap_pt_share> then
> +this option may be used.
> +
> +=item B<enabled>
> +
> +This option enables IOMMU mappings and selects an appropriate default
> +operating mode. For HVM domains running on platforms where the option is
> +available, this is equivalent to B<share_pt>. Otherwise, and also for PV
> +domains, this options is equivalent to B<sync_pt>.

...put the option we want / expect people nearer the top (either first
or second).

Thanks,
 -George

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel