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

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

On Fri, Apr 29, 2016 at 10:12:45AM -0500, Tom Lendacky wrote:
> 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?

I am not saying that you overlooked. Merely wondering if somebody re-orders these
calls what would happen. It maybe also good to have a comment right before
mem_encrpyt_init stating what will happen if the device does DMA before the function
is called.

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