Re: [RFC PATCH] Drivers: hv: Confidential VMBus exernal memory support

[vdso@hexbites.dev]

From: Roman Kisel <vdso@hexbites.dev>

Tianyu Lan wrote:

> In CVM(Confidential VM), system memory is encrypted
> by default. Device drivers typically use the swiotlb
> bounce buffer for DMA memory, which is decrypted
> and shared between the guest and host. Confidential
> Vmbus, however, supports a confidential channel
> that employs encrypted memory for the Vmbus ring
> buffer and external DMA memory. The support for
> the confidential ring buffer has already been
> integrated.
>
> In CVM, device drivers usually employ the standard
> DMA API to map DMA memory with the bounce buffer,
> which remains transparent to the device driver.
> For external DMA memory support, Hyper-V specific
> DMA operations are introduced, bypassing the bounce
> buffer when the confidential external memory flag
> is set. These DMA operations might also be reused
> for TDISP devices in the future, which also support
> DMA operations with encrypted memory.
>
> The DMA operations used are global architecture
> DMA operations (for details, see get_arch_dma_ops()
> and get_dma_ops()), and there is no need to set up
> for each device individually.
>
> Signed-off-by: Tianyu Lan <tiala@microsoft.com>

Tinayu,

Looks great to me!

Reviewed-by: Roman Kisel <vdso@hexbites.dev>

P.S. For the inclined reader, here is how the old, only for
storage and not satisfactory in other ways my attempt to solve this:

https://lore.kernel.org/linux-hyperv/20250409000835.285105-6-romank@linux.microsoft.com/
https://lore.kernel.org/linux-hyperv/20250409000835.285105-7-romank@linux.microsoft.com/

Maybe it'd be a good idea to CC folks who provided feedback back then.

> ---
>  drivers/hv/vmbus_drv.c | 90 +++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 89 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 0dc4692b411a..ca31231b2c32 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -39,6 +39,9 @@
>  #include <clocksource/hyperv_timer.h>
>  #include <asm/mshyperv.h>
>  #include "hyperv_vmbus.h"
> +#include "../../kernel/dma/direct.h"
> +
> +extern const struct dma_map_ops *dma_ops;
>
>  struct vmbus_dynid {
>  	struct list_head node;
> @@ -1429,6 +1432,88 @@ static int vmbus_alloc_synic_and_connect(void)
>  	return -ENOMEM;
>  }
>
> +
> +static bool hyperv_private_memory_dma(struct device *dev)
> +{
> +	struct hv_device *hv_dev = device_to_hv_device(dev);
> +
> +	if (hv_dev && hv_dev->channel && hv_dev->channel->co_external_memory)
> +		return true;
> +	else
> +		return false;
> +}
> +
> +static dma_addr_t hyperv_dma_map_page(struct device *dev, struct page *page,
> +		unsigned long offset, size_t size,
> +		enum dma_data_direction dir,
> +		unsigned long attrs)
> +{
> +	phys_addr_t phys = page_to_phys(page) + offset;
> +
> +	if (hyperv_private_memory_dma(dev))
> +		return __phys_to_dma(dev, phys);
> +	else
> +		return dma_direct_map_phys(dev, phys, size, dir, attrs);
> +}
> +
> +static void hyperv_dma_unmap_page(struct device *dev, dma_addr_t dma_handle,
> +		size_t size, enum dma_data_direction dir, unsigned long attrs)
> +{
> +	if (!hyperv_private_memory_dma(dev))
> +		dma_direct_unmap_phys(dev, dma_handle, size, dir, attrs);
> +}
> +
> +static int hyperv_dma_map_sg(struct device *dev, struct scatterlist *sgl,
> +		int nelems, enum dma_data_direction dir,
> +		unsigned long attrs)
> +{
> +	struct scatterlist *sg;
> +	dma_addr_t dma_addr;
> +	int i;
> +
> +	if (hyperv_private_memory_dma(dev)) {
> +		for_each_sg(sgl, sg, nelems, i) {
> +			dma_addr = __phys_to_dma(dev, sg_phys(sg));
> +			sg_dma_address(sg) = dma_addr;
> +			sg_dma_len(sg) = sg->length;
> +		}
> +
> +		return nelems;
> +	} else {
> +		return dma_direct_map_sg(dev, sgl, nelems, dir, attrs);
> +	}
> +}
> +
> +static void hyperv_dma_unmap_sg(struct device *dev, struct scatterlist *sgl,
> +		int nelems, enum dma_data_direction dir, unsigned long attrs)
> +{
> +	if (!hyperv_private_memory_dma(dev))
> +		dma_direct_unmap_sg(dev, sgl, nelems, dir, attrs);
> +}
> +
> +static int hyperv_dma_supported(struct device *dev, u64 mask)
> +{
> +	dev->coherent_dma_mask = mask;
> +	return 1;
> +}
> +
> +static size_t hyperv_dma_max_mapping_size(struct device *dev)
> +{
> +	if (hyperv_private_memory_dma(dev))
> +		return SIZE_MAX;
> +	else
> +		return swiotlb_max_mapping_size(dev);
> +}
> +
> +const struct dma_map_ops hyperv_dma_ops = {
> +	.map_page               = hyperv_dma_map_page,
> +	.unmap_page             = hyperv_dma_unmap_page,
> +	.map_sg                 = hyperv_dma_map_sg,
> +	.unmap_sg               = hyperv_dma_unmap_sg,
> +	.dma_supported          = hyperv_dma_supported,
> +	.max_mapping_size	= hyperv_dma_max_mapping_size,
> +};
> +
>  /*
>   * vmbus_bus_init -Main vmbus driver initialization routine.
>   *
> @@ -1479,8 +1564,11 @@ static int vmbus_bus_init(void)
>  	 * doing that on each VP while initializing SynIC's wastes time.
>  	 */
>  	is_confidential = ms_hyperv.confidential_vmbus_available;
> -	if (is_confidential)
> +	if (is_confidential) {
> +		dma_ops = &hyperv_dma_ops;
>  		pr_info("Establishing connection to the confidential VMBus\n");
> +	}
> +
>  	hv_para_set_sint_proxy(!is_confidential);
>  	ret = vmbus_alloc_synic_and_connect();
>  	if (ret)
> --
> 2.50.1