Subject: Re: [PATCH 10/13] x86/Swiotlb: Add Swiotlb bounce buffer remap function for HV IVM

From: Konrad Rzeszutek Wilk <[email-masked]>
Date: Thu, 29 Jul 2021 12:29:21 -0400

On Wed, Jul 28, 2021 at 10:52:25AM -0400, Tianyu Lan wrote:
> From: Tianyu Lan <[email-masked]>
> 
> In Isolation VM with AMD SEV, bounce buffer needs to be accessed via
> extra address space which is above shared_gpa_boundary
> (E.G 39 bit address line) reported by Hyper-V CPUID ISOLATION_CONFIG.
> The access physical address will be original physical address +
> shared_gpa_boundary. The shared_gpa_boundary in the AMD SEV SNP
> spec is called virtual top of memory(vTOM). Memory addresses below
> vTOM are automatically treated as private while memory above
> vTOM is treated as shared.
> 
> Use dma_map_decrypted() in the swiotlb code, store remap address returned
> and use the remap address to copy data from/to swiotlb bounce buffer.
> 
> Signed-off-by: Tianyu Lan <[email-masked]>
> ---
>  include/linux/swiotlb.h |  4 ++++
>  kernel/dma/swiotlb.c    | 11 ++++++++---
>  2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index f507e3eacbea..584560ecaa8e 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -72,6 +72,9 @@ extern enum swiotlb_force swiotlb_force;
>   * @end:	The end address of the swiotlb memory pool. Used to do a quick
>   *		range check to see if the memory was in fact allocated by this
>   *		API.
> + * @vaddr:	The vaddr of the swiotlb memory pool. The swiotlb
> + *		memory pool may be remapped in the memory encrypted case and store
> + *		virtual address for bounce buffer operation.
>   * @nslabs:	The number of IO TLB blocks (in groups of 64) between @start and
>   *		@end. For default swiotlb, this is command line adjustable via
>   *		setup_io_tlb_npages.
> @@ -89,6 +92,7 @@ extern enum swiotlb_force swiotlb_force;
>  struct io_tlb_mem {
>  	phys_addr_t start;
>  	phys_addr_t end;
> +	void *vaddr;
>  	unsigned long nslabs;
>  	unsigned long used;
>  	unsigned int index;
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 1fa81c096c1d..6866e5784b53 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -194,8 +194,13 @@ static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,
>  		mem->slots[i].alloc_size = 0;
>  	}
>  
> -	set_memory_decrypted((unsigned long)vaddr, bytes >> PAGE_SHIFT);
> -	memset(vaddr, 0, bytes);
> +	mem->vaddr = dma_map_decrypted(vaddr, bytes);
> +	if (!mem->vaddr) {
> +		pr_err("Failed to decrypt memory.\n");

I am wondering if it would be worth returning an error code in this
function instead of just printing an error?

For this patch I think it is Ok, but perhaps going forward this would be
better done as I am thinking - is there some global guest->hyperv
reporting mechanism so that if this fails - it ends up being bubbled up
to the HyperV console-ish?

And ditto for other hypervisors?

> +		return;
> +	}
> +
> +	memset(mem->vaddr, 0, bytes);
>  }
>  
>  int __init swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose)
> @@ -360,7 +365,7 @@ static void swiotlb_bounce(struct device *dev, phys_addr_t tlb_addr, size_t size
>  	phys_addr_t orig_addr = mem->slots[index].orig_addr;
>  	size_t alloc_size = mem->slots[index].alloc_size;
>  	unsigned long pfn = PFN_DOWN(orig_addr);
> -	unsigned char *vaddr = phys_to_virt(tlb_addr);
> +	unsigned char *vaddr = mem->vaddr + tlb_addr - mem->start;
>  	unsigned int tlb_offset;
>  
>  	if (orig_addr == INVALID_PHYS_ADDR)
> -- 
> 2.25.1
>