Re: [RFC PATCH 10/11] powerpc/svm: Force the use of bounce buffers

Hello Christoph,

Thanks for your quick review.

Christoph Hellwig [off-list ref] writes:

On Thu, Aug 23, 2018 at 11:59:32PM -0300, Thiago Jung Bauermann wrote:

quoted

From: Anshuman Khandual <redacted>

SWIOTLB checks range of incoming CPU addresses to be bounced and see if the
device can access it through it's DMA window without requiring bouncing. In
such cases it just chooses to skip bouncing. But for cases like secure
guests on powerpc platform all addresses need to be bounced into the shared
pool of memory because the host cannot access it otherwise. Hence the need
to do the bouncing is not related to device's DMA window. Hence force the
use of bouncing by setting the swiotlb_force variable on secure guests.

Signed-off-by: Anshuman Khandual <redacted>
Signed-off-by: Thiago Jung Bauermann <redacted>
---
 arch/powerpc/kernel/svm.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/kernel/svm.c b/arch/powerpc/kernel/svm.c
index 1af5caa955f5..f0576ad65cd0 100644
--- a/arch/powerpc/kernel/svm.c
+++ b/arch/powerpc/kernel/svm.c

@@ -17,6 +17,7 @@ static int __init init_svm(void)
 		return 0;

 	ppc_swiotlb_enable = 1;
+	swiotlb_force = SWIOTLB_FORCE;
 	swiotlb_update_mem_attributes();

This needs a comment.

Good point. Will add one.

If in the new scheme of things (with bus_dma_mask?) dma_capable() on a
virtio device running in a secure guest returns false, then this patch
becomes unnecessary and can be dropped.

--
Thiago Jung Bauermann
IBM Linux Technology Center