Re: [RFC PATCH 15/45] KVM: arm64: pkvm: Add __pkvm_host_share/unshare_dma()

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

Hi Tina,

On Sat, Feb 04, 2023 at 08:51:38PM +0800, tina.zhang wrote:
> > +int __pkvm_host_share_dma(phys_addr_t phys_addr, size_t size, bool is_ram)
> > +{
> > +	int i;
> > +	int ret;
> > +	size_t nr_pages = size >> PAGE_SHIFT;
> > +
> > +	if (WARN_ON(!PAGE_ALIGNED(phys_addr | size)))
> > +		return -EINVAL;
> > +
> > +	host_lock_component();
> > +	hyp_lock_component();
> > +
> > +	for (i = 0; i < nr_pages; i++) {
> > +		ret = __pkvm_host_share_dma_page(phys_addr + i * PAGE_SIZE,
> > +						 is_ram);
> Hi Jean,
> 
> I'm not familiar with ARM arch. Just out of curiosity. If pKVM-ARM populates
> the host stage-2 page table lazily, would there be a case that device driver
> in host triggers DMA with pages which have not been mapped to the host
> stage-2 page table yet? How do we handle this situation?

It's possible that the host asks the hypervisor to map on the IOMMU side
a page that is not yet mapped on the CPU side. In general before calling
map_pages() the host zero-initializes the page, triggering a page fault
which creates the mapping, so this case is rare. But if it happens,
__pkvm_host_share_dma() will create the CPU stage-2 mapping:

 __pkvm_host_share_dma()
  do_share()
   host_initiate_share()
    host_stage2_idmap_locked()

which creates a valid identity mapping, along with the ownership
information PVKM_PAGE_SHARED_OWNED. That ownership info is really all we
need here, to prevent future donations to guests or hyp. Since the SMMU
side uses separate stage-2 page tables, we don't actually need to create a
valid mapping on the CPU side yet, that's just how pKVM's mem_protect
currently works. But I don't think it hurts to create the mapping right
away instead of waiting for the CPU page fault, because the host will
likely access the page soon to read what the device wrote there.

Thanks,
Jean

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