[PATCH v2 4/4] drm/ttm: Cache dma pool decrypted pages when AMD SEV is active

Thomas Hellström (VMware) thomas_os at shipmail.org
Tue Sep 3 13:15:04 UTC 2019

From: Thomas Hellstrom <thellstrom at vmware.com>

The TTM dma pool allocates coherent pages for use with TTM. When forcing
unencrypted DMA, such allocations become very expensive since the linear
kernel map has to be changed to mark the pages decrypted. To avoid too many
such allocations and frees, cache the decrypted pages even if they
are in the normal cpu caching state, where otherwise the pool frees them
immediately when unused.

Tested with vmwgfx on SEV-ES.

Cc: Dave Hansen <dave.hansen at linux.intel.com>
Cc: Andy Lutomirski <luto at kernel.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: Borislav Petkov <bp at alien8.de>
Cc: "H. Peter Anvin" <hpa at zytor.com>
Cc: Heiko Carstens <heiko.carstens at de.ibm.com>
Cc: Christian Borntraeger <borntraeger at de.ibm.com>
Cc: Tom Lendacky <thomas.lendacky at amd.com>
Cc: Christian König <christian.koenig at amd.com>
Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>
 drivers/gpu/drm/ttm/ttm_page_alloc_dma.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c b/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
index 9b15df8ecd49..a3247f24e106 100644
--- a/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
+++ b/drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
@@ -1000,7 +1000,7 @@ void ttm_dma_unpopulate(struct ttm_dma_tt *ttm_dma, struct device *dev)
 	struct dma_pool *pool;
 	struct dma_page *d_page, *next;
 	enum pool_type type;
-	bool is_cached = false;
+	bool immediate_free = false;
 	unsigned count, i, npages = 0;
 	unsigned long irq_flags;
@@ -1035,8 +1035,17 @@ void ttm_dma_unpopulate(struct ttm_dma_tt *ttm_dma, struct device *dev)
 	if (!pool)
-	is_cached = (ttm_dma_find_pool(pool->dev,
-		     ttm_to_type(ttm->page_flags, tt_cached)) == pool);
+	/*
+	 * If memory is cached and sev encryption is not active, allocating
+	 * and freeing coherent memory is relatively cheap, so we can free
+	 * it immediately. If sev encryption is active, allocating coherent
+	 * memory involves a call to set_memory_decrypted() which is very
+	 * expensive, so cache coherent pages is sev is active.
+	 */
+	immediate_free = (ttm_dma_find_pool
+			  (pool->dev,
+			   ttm_to_type(ttm->page_flags, tt_cached)) == pool &&
+			  !force_dma_unencrypted(dev));
 	/* make sure pages array match list and count number of pages */
 	count = 0;
@@ -1051,13 +1060,13 @@ void ttm_dma_unpopulate(struct ttm_dma_tt *ttm_dma, struct device *dev)
 			d_page->vaddr &= ~VADDR_FLAG_UPDATED_COUNT;
-		if (is_cached)
+		if (immediate_free)
 			ttm_dma_page_put(pool, d_page);
 	spin_lock_irqsave(&pool->lock, irq_flags);
 	pool->npages_in_use -= count;
-	if (is_cached) {
+	if (immediate_free) {
 		pool->nfrees += count;
 	} else {
 		pool->npages_free += count;

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