[PATCH] swiotlb: Do not zero buffer in set_memory_decrypted()

[PATCH] swiotlb: Do not zero buffer in set_memory_decrypted()

[Kirill A. Shutemov]

For larger TDX VM, memset() after set_memory_decrypted() in
swiotlb_update_mem_attributes() takes substantial portion of boot time.

Zeroing doesn't serve any functional purpose. Malicious VMM can mess
with decrypted/shared buffer at any point.

Remove the memset().

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 kernel/dma/swiotlb.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index f1e7ea160b43..9390b38d2897 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -207,8 +207,6 @@ void __init swiotlb_update_mem_attributes(void)
 	mem->vaddr = swiotlb_mem_remap(mem, bytes);
 	if (!mem->vaddr)
 		mem->vaddr = vaddr;
-
-	memset(mem->vaddr, 0, bytes);
 }
 
 static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,
-- 
2.34.1

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Re: [PATCH] swiotlb: Do not zero buffer in set_memory_decrypted()

[Tom Lendacky via iommu]

On 1/25/22 07:20, Kirill A. Shutemov wrote:
> For larger TDX VM, memset() after set_memory_decrypted() in
> swiotlb_update_mem_attributes() takes substantial portion of boot time.
> 
> Zeroing doesn't serve any functional purpose. Malicious VMM can mess
> with decrypted/shared buffer at any point.
> 
> Remove the memset().
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Acked-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>   kernel/dma/swiotlb.c | 2 --
>   1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index f1e7ea160b43..9390b38d2897 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -207,8 +207,6 @@ void __init swiotlb_update_mem_attributes(void)
>   	mem->vaddr = swiotlb_mem_remap(mem, bytes);
>   	if (!mem->vaddr)
>   		mem->vaddr = vaddr;
> -
> -	memset(mem->vaddr, 0, bytes);
>   }
>   
>   static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,
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Re: [PATCH] swiotlb: Do not zero buffer in set_memory_decrypted()

[Christoph Hellwig]

Thanks,

applied to the dma-mapping for-next tree.
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