RE: [PATCH V2 5/6] net: netvsc: Add Isolation VM support for netvsc driver

[Haiyang Zhang <haiyangz@microsoft.com>]

> -----Original Message-----
> From: Michael Kelley (LINUX) <mikelley@microsoft.com>
> Sent: Wednesday, November 24, 2021 12:03 PM
> To: Tianyu Lan <ltykernel@gmail.com>; tglx@linutronix.de; mingo@redhat.com; bp@alien8.de;
> dave.hansen@linux.intel.com; x86@kernel.org; hpa@zytor.com; luto@kernel.org;
> peterz@infradead.org; jgross@suse.com; sstabellini@kernel.org; boris.ostrovsky@oracle.com;
> KY Srinivasan <kys@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>; Stephen
> Hemminger <sthemmin@microsoft.com>; wei.liu@kernel.org; Dexuan Cui <decui@microsoft.com>;
> joro@8bytes.org; will@kernel.org; davem@davemloft.net; kuba@kernel.org; jejb@linux.ibm.com;
> martin.petersen@oracle.com; hch@lst.de; m.szyprowski@samsung.com; robin.murphy@arm.com;
> Tianyu Lan <Tianyu.Lan@microsoft.com>; thomas.lendacky@amd.com; xen-
> devel@lists.xenproject.org
> Cc: iommu@lists.linux-foundation.org; linux-hyperv@vger.kernel.org; linux-
> kernel@vger.kernel.org; linux-scsi@vger.kernel.org; netdev@vger.kernel.org; vkuznets
> <vkuznets@redhat.com>; brijesh.singh@amd.com; konrad.wilk@oracle.com;
> parri.andrea@gmail.com; dave.hansen@intel.com
> Subject: RE: [PATCH V2 5/6] net: netvsc: Add Isolation VM support for netvsc driver
> 
> From: Tianyu Lan <ltykernel@gmail.com> 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