Re: [PATCH V5 12/12] net: netvsc: Add Isolation VM support for netvsc driver

[Tianyu Lan <ltykernel@gmail.com>]

On 9/16/2021 12:21 AM, Michael Kelley wrote:
> I think you are proposing this approach to allocating memory for the send
> and receive buffers so that you can avoid having two virtual mappings for
> the memory, per comments from Christop Hellwig.  But overall, the approach
> seems a bit complex and I wonder if it is worth it.  If allocating large contiguous
> chunks of physical memory is successful, then there is some memory savings
> in that the data structures needed to keep track of the physical pages is
> smaller than the equivalent page tables might be.  But if you have to revert
> to allocating individual pages, then the memory savings is reduced.
> 

Yes, this version follows idea from Christop in the previous 
discussion.(https://lkml.org/lkml/2021/9/2/112)
This patch shows the implementation and check whether this is a right 
direction.

> Ultimately, the list of actual PFNs has to be kept somewhere.  Another approach
> would be to do the reverse of what hv_map_memory() from the v4 patch
> series does.  I.e., you could do virt_to_phys() on each virtual address that
> maps above VTOM, and subtract out the shared_gpa_boundary to get the
> list of actual PFNs that need to be freed.

virt_to_phys() doesn't work for virtual address returned by 
vmap/vmap_pfn() (just like it doesn't work for va returned by 
vmalloc()). The pfn above vTom doesn't have struct page backing and
vmap_pfn() populates the pfn directly in the pte.(Please see the
vmap_pfn_apply()). So it's not easy to convert the va to pa.

>   This way you don't have two copies
> of the list of PFNs -- one with and one without the shared_gpa_boundary added.
> But it comes at the cost of additional code so that may not be a great idea.
> 
> I think what you have here works, and I don't have a clearly better solution
> at the moment except perhaps to revert to the v4 solution and just have two
> virtual mappings.  I'll keep thinking about it.  Maybe Christop has other
> thoughts.