linux-kernel - Re: [PATCH v3 13/16] powerpc/pseries/iommu: Don't use dma_iommu

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Message-ID: <87k1bpzrfl.fsf@morokweng.localdomain>
Date:   Tue, 06 Aug 2019 22:37:34 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Anshuman Khandual <anshuman.linux@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Mike Anderson <andmike@...ux.ibm.com>,
        Paul Mackerras <paulus@...ba.org>,
        Ram Pai <linuxram@...ibm.com>,
        Claudio Carvalho <cclaudio@...ux.ibm.com>
Subject: Re: [PATCH v3 13/16] powerpc/pseries/iommu: Don't use dma_iommu_ops on secure guests


Hello Christoph,

Thanks for your review.

Christoph Hellwig <hch@....de> writes:

> On Tue, Aug 06, 2019 at 02:22:34AM -0300, Thiago Jung Bauermann wrote:
>> @@ -1318,7 +1319,10 @@ void iommu_init_early_pSeries(void)
>>  	of_reconfig_notifier_register(&iommu_reconfig_nb);
>>  	register_memory_notifier(&iommu_mem_nb);
>>  
>> -	set_pci_dma_ops(&dma_iommu_ops);
>> +	if (is_secure_guest())
>> +		set_pci_dma_ops(NULL);
>> +	else
>> +		set_pci_dma_ops(&dma_iommu_ops);
>
> Shoudn't:
>
> 	if (!is_secure_guest())
> 		set_pci_dma_ops(&dma_iommu_ops);
>
> be enough here, given that NULL is the default?

Indeed, it is enough.

> Also either way I think this conditional needs a comment explaining
> why it is there.

Good point. I added the commit message as a comment in the code.
New version of this patch below.


>From 5dc3914efa4765eef2869d554d4ea1c676bb1e75 Mon Sep 17 00:00:00 2001
From: Thiago Jung Bauermann <bauerman@...ux.ibm.com>
Date: Thu, 24 Jan 2019 22:40:16 -0200
Subject: [PATCH] powerpc/pseries/iommu: Don't use dma_iommu_ops on secure
 guests

Secure guest memory is inacessible to devices so regular DMA isn't
possible.

In that case set devices' dma_map_ops to NULL so that the generic
DMA code path will use SWIOTLB to bounce buffers for DMA.

Signed-off-by: Thiago Jung Bauermann <bauerman@...ux.ibm.com>
---
 arch/powerpc/platforms/pseries/iommu.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
index 889dc2e44b89..8d9c2b17ad54 100644
--- a/arch/powerpc/platforms/pseries/iommu.c
+++ b/arch/powerpc/platforms/pseries/iommu.c
@@ -36,6 +36,7 @@
 #include <asm/udbg.h>
 #include <asm/mmzone.h>
 #include <asm/plpar_wrappers.h>
+#include <asm/svm.h>
 
 #include "pseries.h"
 
@@ -1318,7 +1319,15 @@ void iommu_init_early_pSeries(void)
 	of_reconfig_notifier_register(&iommu_reconfig_nb);
 	register_memory_notifier(&iommu_mem_nb);
 
-	set_pci_dma_ops(&dma_iommu_ops);
+	/*
+	 * Secure guest memory is inacessible to devices so regular DMA isn't
+	 * possible.
+	 *
+	 * In that case keep devices' dma_map_ops as NULL so that the generic
+	 * DMA code path will use SWIOTLB to bounce buffers for DMA.
+	 */
+	if (!is_secure_guest())
+		set_pci_dma_ops(&dma_iommu_ops);
 }
 
 static int __init disable_multitce(char *str)