Re: [Xen-devel] [RFC PATCH 9/9] xen: Add use_iommu flag to createdomain domctl

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.03.17 at 17:36, <olekstysh@xxxxxxxxx> wrote:
> On Wed, Mar 22, 2017 at 5:56 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>> On 15.03.17 at 21:05, <olekstysh@xxxxxxxxx> wrote:
>>> --- a/xen/include/public/domctl.h
>>> +++ b/xen/include/public/domctl.h
>>> @@ -66,6 +66,9 @@ struct xen_domctl_createdomain {
>>>   /* Is this a xenstore domain? */
>>>  #define _XEN_DOMCTL_CDF_xs_domain     5
>>>  #define XEN_DOMCTL_CDF_xs_domain      (1U<<_XEN_DOMCTL_CDF_xs_domain)
>>> + /* Should IOMMU page tables be populated at the domain creation time? */
>>> +#define _XEN_DOMCTL_CDF_use_iommu     6
>>> +#define XEN_DOMCTL_CDF_use_iommu      (1U<<_XEN_DOMCTL_CDF_use_iommu)
>>>      uint32_t flags;
>>
>> The need for this to be done via domain creation flag (rather than
>> as a separate, later step) needs to be well explained. Generally
>> what to add here should only be things which can't be done later
>> in a reasonable way.
> 
> Well, the non-shared IOMMU should populate its page table by the time
> the P2M code will have started update mappings. Theoretically, it
> might happen right after p2m_init has been completed,
> that is, even during createdomain domctl execution. For example, I see
> that domain_vgic_init() inserts mapping to P2M table, because of
> map_mmio_regions() is being called during VGIC initialization.
> Not sure that GIC mmio ranges must be present in the IOMMU page table,
> but anyway, it might be the per-domain initialization of other IPs,
> co-processors that mapping mustn't be skipped because of IOMMU is not
> ready.
> So, as both iommu_domain_init() and p2m_init() are called from
> arch_domain_create(), i.e. during createdomain domctl execution, we
> have to know exactly should we use IOMMU for this domain to pass
> proper value to iommu_domain_init().

Well, no, iommu_domain_init() is not supposed to do any table
setup, so it shouldn't need to know.

> If don't take into account everything I wrote above, yes, it is
> possible to introduce new domctl for this purpose that will be called
> later, but before any operations with guest_physmap.

Exactly. Any other things needing syncing, but being done during
domain creation may then need syncing over. One might question
whether some of those things then actually are being done too
early (and quite possibly have been done that way just for simplicity).

Jan


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