Re: [RFC v2 1/2] powerpc/pseries/iommu: Share the per-cpu TCE page with the hypervisor.
From: David Gibson <hidden>
Date: 2019-11-10 21:10:55
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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.
quoted hunk ↗ jump to hunk
Signed-off-by: Ram Pai <redacted> --- 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
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