swiotlb: do not zero buffer in set_memory_decrypted()

kiryl · Christoph Hellwig · commit dfcf2e017f5b · 2022-01-26T17:05:54.000+01:00
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 &lt;kirill.shutemov@linux.intel.com&gt;
Acked-by: Tom Lendacky &lt;thomas.lendacky@amd.com&gt;
Signed-off-by: Christoph Hellwig &lt;hch@lst.de&gt;
diff --git 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,

Original file line number	Diff line number	Diff line change
`@@ -207,8 +207,6 @@ void __init swiotlb_update_mem_attributes(void)`
`207`	`207`	`mem->vaddr = swiotlb_mem_remap(mem, bytes);`
`208`	`208`	`if (!mem->vaddr)`
`209`	`209`	`mem->vaddr = vaddr;`
`210`		`-`
`211`		`- memset(mem->vaddr, 0, bytes);`
`212`	`210`	`}`
`213`	`211`
`214`	`212`	`static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,`