Re: [Resend RFC PATCH V2 09/12] swiotlb: Add bounce buffer remap address setting function

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Apr 14, 2021 at 10:49:42AM -0400, Tianyu Lan wrote:
> From: Tianyu Lan <Tianyu.Lan@microsoft.com>
> 
> For Hyper-V isolation VM with AMD SEV SNP, the bounce buffer(shared memory)
> needs to be accessed via extra address space(e.g address above bit39).
> Hyper-V code may remap extra address space outside of swiotlb. swiotlb_bounce()

May? Isn't this a MUST in this case?

> needs to use remap virtual address to copy data from/to bounce buffer. Add
> new interface swiotlb_set_bounce_remap() to do that.

I am bit lost - why can't you use the swiotlb_init and pass in your
DMA pool that is already above bit 39?

> 
> Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
> ---
>  include/linux/swiotlb.h |  5 +++++
>  kernel/dma/swiotlb.c    | 13 ++++++++++++-
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index d9c9fc9ca5d2..3ccd08116683 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -82,8 +82,13 @@ unsigned int swiotlb_max_segment(void);
>  size_t swiotlb_max_mapping_size(struct device *dev);
>  bool is_swiotlb_active(void);
>  void __init swiotlb_adjust_size(unsigned long new_size);
> +void swiotlb_set_bounce_remap(unsigned char *vaddr);
>  #else
>  #define swiotlb_force SWIOTLB_NO_FORCE
> +static inline void swiotlb_set_bounce_remap(unsigned char *vaddr)
> +{
> +}
> +
>  static inline bool is_swiotlb_buffer(phys_addr_t paddr)
>  {
>  	return false;
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 7c42df6e6100..5fd2db6aa149 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -94,6 +94,7 @@ static unsigned int io_tlb_index;
>   * not be bounced (unless SWIOTLB_FORCE is set).
>   */
>  static unsigned int max_segment;
> +static unsigned char *swiotlb_bounce_remap_addr;
>  
>  /*
>   * We need to save away the original address corresponding to a mapped entry
> @@ -421,6 +422,11 @@ void __init swiotlb_exit(void)
>  	swiotlb_cleanup();
>  }
>  
> +void swiotlb_set_bounce_remap(unsigned char *vaddr)
> +{
> +	swiotlb_bounce_remap_addr = vaddr;
> +}
> +
>  /*
>   * Bounce: copy the swiotlb buffer from or back to the original dma location
>   */
> @@ -428,7 +434,12 @@ static void swiotlb_bounce(phys_addr_t orig_addr, phys_addr_t tlb_addr,
>  			   size_t size, enum dma_data_direction dir)
>  {
>  	unsigned long pfn = PFN_DOWN(orig_addr);
> -	unsigned char *vaddr = phys_to_virt(tlb_addr);
> +	unsigned char *vaddr;
> +
> +	if (swiotlb_bounce_remap_addr)
> +		vaddr = swiotlb_bounce_remap_addr + tlb_addr - io_tlb_start;
> +	else
> +		vaddr = phys_to_virt(tlb_addr);
>  
>  	if (PageHighMem(pfn_to_page(pfn))) {
>  		/* The buffer does not have a mapping.  Map it in and copy */
> -- 
> 2.25.1
>