Re: [RFC PATCH v1 13/18] x86: DMA support for memory encryption

[Tom Lendacky]

On 04/29/2016 02:17 AM, Konrad Rzeszutek Wilk wrote:
> On Tue, Apr 26, 2016 at 05:58:12PM -0500, Tom Lendacky wrote:
>> Since DMA addresses will effectively look like 48-bit addresses when the
>> memory encryption mask is set, SWIOTLB is needed if the DMA mask of the
>> device performing the DMA does not support 48-bits. SWIOTLB will be
>> initialized to create un-encrypted bounce buffers for use by these devices.
>>
> 
> 
> I presume the sme_me_mask does not use the lower 48 bits?

The sme_me_mask will actually be bit 47. So, when applied, the address
will become a 48-bit address.

> 
> 
> ..snip..
>> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
>> index 7d56d1b..594dc65 100644
>> --- a/arch/x86/mm/mem_encrypt.c
>> +++ b/arch/x86/mm/mem_encrypt.c
>> @@ -12,6 +12,8 @@
>>  
>>  #include <linux/init.h>
>>  #include <linux/mm.h>
>> +#include <linux/dma-mapping.h>
>> +#include <linux/swiotlb.h>
>>  
>>  #include <asm/mem_encrypt.h>
>>  #include <asm/cacheflush.h>
>> @@ -168,6 +170,25 @@ void __init sme_early_init(void)
>>  }
>>  
>>  /* Architecture __weak replacement functions */
>> +void __init mem_encrypt_init(void)
>> +{
>> +	if (!sme_me_mask)
>> +		return;
>> +
>> +	/* Make SWIOTLB use an unencrypted DMA area */
>> +	swiotlb_clear_encryption();
>> +}
>> +
>> +unsigned long swiotlb_get_me_mask(void)
>> +{
>> +	return sme_me_mask;
>> +}
>> +
>> +void swiotlb_set_mem_dec(void *vaddr, unsigned long size)
>> +{
>> +	sme_set_mem_dec(vaddr, size);
>> +}
>> +
>>  void __init *efi_me_early_memremap(resource_size_t paddr,
>>  				   unsigned long size)
>>  {
>> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
>> index 017fced..121b9de 100644
>> --- a/include/linux/swiotlb.h
>> +++ b/include/linux/swiotlb.h
>> @@ -30,6 +30,7 @@ int swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose);
>>  extern unsigned long swiotlb_nr_tbl(void);
>>  unsigned long swiotlb_size_or_default(void);
>>  extern int swiotlb_late_init_with_tbl(char *tlb, unsigned long nslabs);
>> +extern void __init swiotlb_clear_encryption(void);
>>  
>>  /*
>>   * Enumeration for sync targets
>> diff --git a/init/main.c b/init/main.c
>> index b3c6e36..1013d1c 100644
>> --- a/init/main.c
>> +++ b/init/main.c
>> @@ -458,6 +458,10 @@ void __init __weak thread_info_cache_init(void)
>>  }
>>  #endif
>>  
>> +void __init __weak mem_encrypt_init(void)
>> +{
>> +}
>> +
>>  /*
>>   * Set up kernel memory allocators
>>   */
>> @@ -597,6 +601,8 @@ asmlinkage __visible void __init start_kernel(void)
>>  	 */
>>  	locking_selftest();
>>  
>> +	mem_encrypt_init();
>> +
>>  #ifdef CONFIG_BLK_DEV_INITRD
>>  	if (initrd_start && !initrd_below_start_ok &&
>>  	    page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) {
> 
> What happens if devices use the bounce buffer before mem_encrypt_init()?

The call to mem_encrypt_init is early in the boot process, I may have
overlooked something, but what devices would be performing DMA before
this?

Thanks,
Tom

> 
> ..snip..
>