> -----Original Message-----
> From: Michael Kelley (LINUX) <mikelley@xxxxxxxxxxxxx>
> Sent: Wednesday, November 24, 2021 12:03 PM
> To: Tianyu Lan <ltykernel@xxxxxxxxx>; tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; bp@xxxxxxxxx;
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> KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; Stephen
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> martin.petersen@xxxxxxxxxx; hch@xxxxxx; m.szyprowski@xxxxxxxxxxx; robin.murphy@xxxxxxx;
> Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>; thomas.lendacky@xxxxxxx; xen-
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> Subject: RE: [PATCH V2 5/6] net: netvsc: Add Isolation VM support for netvsc driver
>
> From: Tianyu Lan <ltykernel@xxxxxxxxx> Sent: Tuesday, November 23, 2021 6:31 AM
> >
> > In Isolation VM, all shared memory with host needs to mark visible to
> > host via hvcall. vmbus_establish_gpadl() has already done it for
> > netvsc rx/tx ring buffer. The page buffer used by vmbus_sendpacket_
> > pagebuffer() stills need to be handled. Use DMA API to map/umap these
> > memory during sending/receiving packet and Hyper-V swiotlb bounce
> > buffer dma address will be returned. The swiotlb bounce buffer has
> > been masked to be visible to host during boot up.
> >
> > Allocate rx/tx ring buffer via dma_alloc_noncontiguous() in Isolation
> > VM. After calling vmbus_establish_gpadl() which marks these pages
> > visible to host, map these pages unencrypted addes space via dma_vmap_noncontiguous().
> >
>
> The big unresolved topic is how best to do the allocation and mapping of the big netvsc
> send and receive buffers. Let me summarize and make a recommendation.
>
> Background
> ==========
> 1. Each Hyper-V synthetic network device requires a large pre-allocated receive
> buffer (defaults to 16 Mbytes) and a similar send buffer (defaults to 1 Mbyte).
> 2. The buffers are allocated in guest memory and shared with the Hyper-V host.
> As such, in the Hyper-V SNP environment, the memory must be unencrypted
> and accessed in the Hyper-V guest with shared_gpa_boundary (i.e., VTOM)
> added to the physical memory address.
> 3. The buffers need *not* be contiguous in guest physical memory, but must be
> contiguously mapped in guest kernel virtual space.
> 4. Network devices may come and go during the life of the VM, so allocation of
> these buffers and their mappings may be done after Linux has been running for
> a long time.
> 5. Performance of the allocation and mapping process is not an issue since it is
> done only on synthetic network device add/remove.
> 6. So the primary goals are an appropriate logical abstraction, code that is
> simple and straightforward, and efficient memory usage.
>
> Approaches
> ==========
> During the development of these patches, four approaches have been
> implemented:
>
> 1. Two virtual mappings: One from vmalloc() to allocate the guest memory, and
> the second from vmap_pfns() after adding the shared_gpa_boundary. This is
> implemented in Hyper-V or netvsc specific code, with no use of DMA APIs.
> No separate list of physical pages is maintained, so for creating the second
> mapping, the PFN list is assembled temporarily by doing virt-to-phys()
> page-by-page on the vmalloc mapping, and then discarded because it is no
> longer needed. [v4 of the original patch series.]
>
> 2. Two virtual mappings as in (1) above, but implemented via new DMA calls
> dma_map_decrypted() and dma_unmap_encrypted(). [v3 of the original
> patch series.]
>
> 3. Two virtual mappings as in (1) above, but implemented via DMA noncontiguous
> allocation and mapping calls, as enhanced to allow for custom map/unmap
> implementations. A list of physical pages is maintained in the dma_sgt_handle
> as expected by the DMA noncontiguous API. [New split-off patch series v1 & v2]
>
> 4. Single virtual mapping from vmap_pfns(). The netvsc driver allocates physical
> memory via alloc_pages() with as much contiguity as possible, and maintains a
> list of physical pages and ranges. Single virtual map is setup with vmap_pfns()
> after adding shared_gpa_boundary. [v5 of the original patch series.]
>
> Both implementations using DMA APIs use very little of the existing DMA machinery. Both
> require extensions to the DMA APIs, and custom ops functions.
> While in some sense the netvsc send and receive buffers involve DMA, they do not require
> any DMA actions on a per-I/O basis. It seems better to me to not try to fit these two
> buffers into the DMA model as a one-off. Let's just use Hyper-V specific code to allocate
> and map them, as is done with the Hyper-V VMbus channel ring buffers.
>
> That leaves approaches (1) and (4) above. Between those two, (1) is simpler even though
> there are two virtual mappings. Using alloc_pages() as in (4) is messy and there's no
> real benefit to using higher order allocations.
> (4) also requires maintaining a separate list of PFNs and ranges, which offsets some of
> the benefits to having only one virtual mapping active at any point in time.
>
> I don't think there's a clear "right" answer, so it's a judgment call. We've explored
> what other approaches would look like, and I'd say let's go with
> (1) as the simpler approach. Thoughts?
>
I agree with the following goal:
"So the primary goals are an appropriate logical abstraction, code that is
simple and straightforward, and efficient memory usage."
And the Approach #1 looks better to me as well.
Thanks,
- Haiyang
