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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <3bd96555-ac06-f66a-6c0f-a84c3987c9ca@redhat.com>
Date: Mon, 12 Mar 2018 10:53:01 +0100
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Subject: Re: [Qemu-devel] [PATCH v2] hw/i386: make IOMMUs configurable via
 default-configs/
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To: Thomas Huth <thuth@redhat.com>, qemu-devel@nongnu.org

On 12/03/2018 10:48, Thomas Huth wrote:
>> +obj-$(CONFIG_VTD) += x86-iommu.o intel_iommu.o
>> +obj-$(CONFIG_AMD_IOMMU) += x86-iommu.o amd_iommu.o
> The linker likely does not care if x86-iommu.o is included twice

The linker actually cares, but rules.mak drops the duplicate:

    commit e2a99ad3e174ab4c9d2320dcecd779230409829f
    Author: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
    Date:   Thu Aug 25 09:18:52 2011 +0100

    build: sort objects to remove duplicates for link

    Avoid duplicate object files during the link.  There are legitimate
    cases where a link command-line would include duplicate object files
    because two independent subsystems both depend on common
    infrastructure.

    Use GNU make's $(sort) function to remove duplicate object files
    from the link command-line.

    Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
    Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

> , but wouldn't it be clearer to use this instead:
> 
> obj-$(call lor,$(CONFIG_VTD),$(CONFIG_AMD_IOMMU)) += x86-iommu.o

Possibly, but it doesn't scale too well when we add a third IOMMU.  The
right solution would be something like "select" in Kconfig.  Let me know
what you prefer for x86-iommu.c between obj-y and specifying the file
twice, and I'll do it.

Paolo