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

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Message-Id: <87o9drxrld.fsf@morokweng.localdomain>
Date:   Fri, 24 Aug 2018 20:38:54 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linuxppc-dev@...ts.ozlabs.org, iommu@...ts.linux-foundation.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>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 10/11] powerpc/svm: Force the use of bounce buffers


Hello Christoph,

Thanks for your quick review.

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

> On Thu, Aug 23, 2018 at 11:59:32PM -0300, Thiago Jung Bauermann wrote:
>> From: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
>>
>> 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 <khandual@...ux.vnet.ibm.com>
>> Signed-off-by: Thiago Jung Bauermann <bauerman@...ux.ibm.com>
>> ---
>>  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