On Fri, Nov 08, 2019 at 03:00:10PM -0800, Ram Pai wrote:
> The hypervisor needs to access the contents of the page holding the TCE
> entries while setting up the TCE entries in the IOMMU's TCE table.
> 
> For SecureVMs, since this page is encrypted, the hypervisor cannot
> access valid entries. Share the page with the hypervisor. This ensures
> that the hypervisor sees those valid entries.
> 
> Why is this safe?
>    The page contains only TCE entries; not any sensitive data
>    belonging to the Secure VM. The hypervisor has a genuine need to know
>    the value of the TCE entries, without which it will not be able to
>    DMA to/from the pages pointed to by the TCE entries. In a Secure
>    VM the TCE entries point to pages that are also shared with the
>    hypervisor; example: pages containing bounce buffers.

The bit that may not be obvious to reviewers from the above is this:

This is *not* a page of "live" TCEs which are actively used for
translation.  Instead this is just a transient buffer with a batch of
TCEs to set, passed to the hypervisor with the H_PUT_TCE_INDIRECT call.

> 
> Signed-off-by: Ram Pai <linuxram@us.ibm.com>
> ---
>  arch/powerpc/platforms/pseries/iommu.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index 8d9c2b1..a302aaa 100644
> --- a/arch/powerpc/platforms/pseries/iommu.c
> +++ b/arch/powerpc/platforms/pseries/iommu.c
> @@ -37,6 +37,7 @@
>  #include <asm/mmzone.h>
>  #include <asm/plpar_wrappers.h>
>  #include <asm/svm.h>
> +#include <asm/ultravisor.h>
>  
>  #include "pseries.h"
>  
> @@ -179,6 +180,23 @@ static int tce_build_pSeriesLP(struct iommu_table *tbl, long tcenum,
>  
>  static DEFINE_PER_CPU(__be64 *, tce_page);
>  
> +/*
> + * Allocate a tce page.  If secure VM, share the page with the hypervisor.
> + *
> + * NOTE: the TCE page is shared with the hypervisor explicitly and remains
> + * shared for the lifetime of the kernel. It is implicitly unshared at kernel
> + * shutdown through a UV_UNSHARE_ALL_PAGES ucall.
> + */
> +static __be64 *alloc_tce_page(void)
> +{
> +	__be64 *tcep = (__be64 *)__get_free_page(GFP_ATOMIC);
> +
> +	if (tcep && is_secure_guest())
> +		uv_share_page(PHYS_PFN(__pa(tcep)), 1);
> +
> +	return tcep;
> +}
> +
>  static int tce_buildmulti_pSeriesLP(struct iommu_table *tbl, long tcenum,
>  				     long npages, unsigned long uaddr,
>  				     enum dma_data_direction direction,
> @@ -206,8 +224,7 @@ static int tce_buildmulti_pSeriesLP(struct iommu_table *tbl, long tcenum,
>  	 * from iommu_alloc{,_sg}()
>  	 */
>  	if (!tcep) {
> -		tcep = (__be64 *)__get_free_page(GFP_ATOMIC);
> -		/* If allocation fails, fall back to the loop implementation */
> +		tcep = alloc_tce_page();
>  		if (!tcep) {
>  			local_irq_restore(flags);
>  			return tce_build_pSeriesLP(tbl, tcenum, npages, uaddr,
> @@ -405,7 +422,7 @@ static int tce_setrange_multi_pSeriesLP(unsigned long start_pfn,
>  	tcep = __this_cpu_read(tce_page);
>  
>  	if (!tcep) {
> -		tcep = (__be64 *)__get_free_page(GFP_ATOMIC);
> +		tcep = alloc_tce_page();
>  		if (!tcep) {
>  			local_irq_enable();
>  			return -ENOMEM;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson