Re: [Xen-devel] [PATCH] x86/mtrr: recalculate P2M type for domains with iocaps

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.04.19 at 13:37, <igor.druzhinin@xxxxxxxxxx> wrote:
> This change reflects the logic in epte_get_entry_emt() and allows
> changes in guest MTTRs to be reflected in EPT for domains having
> direct access to certain hardware memory regions but without IOMMU
> context assigned (e.g. XenGT).
> 
> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>

Fundamentally I'm happy to get both in sync, so
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

But I have a question:

>  void memory_type_changed(struct domain *d)
>  {
> -    if ( has_iommu_pt(d) && d->vcpu && d->vcpu[0] )
> +    if ( (has_iommu_pt(d) || cache_flush_permitted(d)) && d->vcpu && 
> d->vcpu[0] )
>      {
>          p2m_memory_type_changed(d);
>          flush_all(FLUSH_CACHE);

Wouldn't cache_flush_permitted() alone suffice, both here and
there? Even if anyone was to pass-through a device without any
MMIO or I/O port BAR, the memory type (which is a CPU side
thing only, i.e. doesn't affect DMA by the device) shouldn't
matter in that case (leaving aside the fact that a BAR-less
device is unlikely to be DMA-capable, unless the programming
of the DMA operations was to happen through vendor specific
config space accesses).

Jan



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