Subject: Re: [RFC PATCH V3 10/11] HV/Netvsc: Add Isolation VM support for netvsc driver

From: Christoph Hellwig <[email-masked]>
Date: Mon, 7 Jun 2021 08:50:07 +0200

On Sun, May 30, 2021 at 11:06:27AM -0400, Tianyu Lan wrote:
> +	if (hv_isolation_type_snp()) {
> +		pfns = kcalloc(buf_size / HV_HYP_PAGE_SIZE, sizeof(unsigned long),
> +			       GFP_KERNEL);
> +		for (i = 0; i < buf_size / HV_HYP_PAGE_SIZE; i++)
> +			pfns[i] = virt_to_hvpfn(net_device->recv_buf + i * HV_HYP_PAGE_SIZE) +
> +				(ms_hyperv.shared_gpa_boundary >> HV_HYP_PAGE_SHIFT);
> +
> +		vaddr = vmap_pfn(pfns, buf_size / HV_HYP_PAGE_SIZE, PAGE_KERNEL_IO);
> +		kfree(pfns);
> +		if (!vaddr)
> +			goto cleanup;
> +		net_device->recv_original_buf = net_device->recv_buf;
> +		net_device->recv_buf = vaddr;
> +	}

This probably wnats a helper to make the thing more readable.  But who
came up with this fucked up communication protocol where the host needs
to map random pfns into a contigous range?  Sometime I really have to
wonder what crack the hyper-v people take when comparing this to the
relatively sane approach others take.

> +	for (i = 0; i < page_count; i++)
> +		dma_unmap_single(&hv_dev->device, packet->dma_range[i].dma,
> +				 packet->dma_range[i].mapping_size,
> +				 DMA_TO_DEVICE);
> +
> +	kfree(packet->dma_range);

Any reason this isn't simply using a struct scatterlist?

> +	for (i = 0; i < page_count; i++) {
> +		char *src = phys_to_virt((pb[i].pfn << HV_HYP_PAGE_SHIFT)
> +					 + pb[i].offset);
> +		u32 len = pb[i].len;
> +
> +		dma = dma_map_single(&hv_dev->device, src, len,
> +				     DMA_TO_DEVICE);

dma_map_single can only be used on page baked memory, and if this is
using page backed memory you wouldn't need to do thee phys_to_virt
tricks.  Can someone explain the mess here in more detail?

>  	struct rndis_device *dev = nvdev->extension;
>  	struct rndis_request *request = NULL;
> +	struct hv_device *hv_dev = ((struct net_device_context *)
> +			netdev_priv(ndev))->device_ctx;

Why not use a net_device_context local variable instead of this cast
galore?