Re: [Xen-devel] [PATCH-for-4.13] x86/mm: don't needlessly veto migration

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.10.2019 10:28, Paul Durrant wrote:
> Now that xl.cfg has an option to explicitly enable IOMMU mappings for a
> domain, migration may be needlessly vetoed due to the check of
> is_iommu_enabled() in paging_log_dirty_enable().
> There is actually no need to prevent logdirty from being enabled unless
> devices are assigned to a domain and that domain is sharing HAP mappings
> with the IOMMU (in which case disabling write permissions in the P2M may
> cause DMA faults).

But that's taking into account only half of the reason of the
exclusion. The other half is that assigned devices may cause pages
to be dirtied behind the back of the log-dirty logic. Therefore ...

> --- a/xen/arch/x86/mm/paging.c
> +++ b/xen/arch/x86/mm/paging.c
> @@ -209,15 +209,15 @@ static int paging_free_log_dirty_bitmap(struct domain 
> *d, int rc)
>      return rc;
>  }
>  
> -int paging_log_dirty_enable(struct domain *d, bool_t log_global)
> +int paging_log_dirty_enable(struct domain *d, bool log_global)
>  {
>      int ret;
>  
> -    if ( is_iommu_enabled(d) && log_global )
> +    if ( has_arch_pdevs(d) && iommu_use_hap_pt(d) && log_global )

... the iommu_use_hap_pt(d) needs to be dropped again, I think.

> --- a/xen/drivers/passthrough/pci.c
> +++ b/xen/drivers/passthrough/pci.c
> @@ -1486,11 +1486,15 @@ static int assign_device(struct domain *d, u16 seg, 
> u8 bus, u8 devfn, u32 flag)
>      if ( !is_iommu_enabled(d) )
>          return 0;
>  
> -    /* Prevent device assign if mem paging or mem sharing have been 
> -     * enabled for this domain */
> +    /*
> +     * Prevent device assign if mem paging or mem sharing have been
> +     * enabled for this domain, or logdirty is enabled and the P2M is
> +     * shared with the IOMMU.
> +     */
>      if ( unlikely(d->arch.hvm.mem_sharing_enabled ||
>                    vm_event_check_ring(d->vm_event_paging) ||
> -                  p2m_get_hostp2m(d)->global_logdirty) )
> +                  (p2m_get_hostp2m(d)->global_logdirty &&
> +                   iommu_use_hap_pt(d))) )

This change wants dropping altogether then.

Jan

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