Platforms that optionally provide an IOMMU protocol should do so by
including a DXE driver (usually called IoMmuDxe) that produces either
the IOMMU protocol -- if the underlying capabilities are available --,
or gIoMmuAbsentProtocolGuid, to signal that the IOMMU capability
detection completed with negative result (i.e., no IOMMU will be
available in the system).

In turn, DXE drivers (and library instances) that are supposed to use
the IOMMU protocol if it is available should add the following to
their DEPEX:

gEdkiiIoMmuProtocolGuid OR gIoMmuAbsentProtocolGuid

This ensures these client modules will only be dispatched after IOMMU
detection completes (with positive or negative result).

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Leo Duran <leo.duran@amd.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
 OvmfPkg/OvmfPkg.dec | 1 +
 1 file changed, 1 insertion(+)

diff --git a/OvmfPkg/OvmfPkg.dec b/OvmfPkg/OvmfPkg.dec
index 5627be0bab0a..4241af10c3d0 100644
--- a/OvmfPkg/OvmfPkg.dec
+++ b/OvmfPkg/OvmfPkg.dec
@@ -69,6 +69,7 @@ [Protocols]
   gBlockMmioProtocolGuid              = {0x6b558ce3, 0x69e5, 0x4c67, {0xa6, 0x34, 0xf7, 0xfe, 0x72, 0xad, 0xbe, 0x84}}
   gXenBusProtocolGuid                 = {0x3d3ca290, 0xb9a5, 0x11e3, {0xb7, 0x5d, 0xb8, 0xac, 0x6f, 0x7d, 0x65, 0xe6}}
   gXenIoProtocolGuid                  = {0x6efac84f, 0x0ab0, 0x4747, {0x81, 0xbe, 0x85, 0x55, 0x62, 0x59, 0x04, 0x49}}
+  gIoMmuAbsentProtocolGuid            = {0xf8775d50, 0x8abd, 0x4adf, {0x92, 0xac, 0x85, 0x3e, 0x51, 0xf6, 0xc8, 0xdc}}
 
 [PcdsFixedAtBuild]
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfPeiMemFvBase|0x0|UINT32|0
-- 
2.7.4

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On 06/22/17 22:37, Brijesh Singh wrote:
> Platforms that optionally provide an IOMMU protocol should do so by
> including a DXE driver (usually called IoMmuDxe) that produces either
> the IOMMU protocol -- if the underlying capabilities are available --,
> or gIoMmuAbsentProtocolGuid, to signal that the IOMMU capability
> detection completed with negative result (i.e., no IOMMU will be
> available in the system).
> 
> In turn, DXE drivers (and library instances) that are supposed to use
> the IOMMU protocol if it is available should add the following to
> their DEPEX:
> 
> gEdkiiIoMmuProtocolGuid OR gIoMmuAbsentProtocolGuid
> 
> This ensures these client modules will only be dispatched after IOMMU
> detection completes (with positive or negative result).
> 
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Leo Duran <leo.duran@amd.com>
> Contributed-under: TianoCore Contribution Agreement 1.0
> Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
> Suggested-by: Laszlo Ersek <lersek@redhat.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
>  OvmfPkg/OvmfPkg.dec | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/OvmfPkg/OvmfPkg.dec b/OvmfPkg/OvmfPkg.dec
> index 5627be0bab0a..4241af10c3d0 100644
> --- a/OvmfPkg/OvmfPkg.dec
> +++ b/OvmfPkg/OvmfPkg.dec
> @@ -69,6 +69,7 @@ [Protocols]
>    gBlockMmioProtocolGuid              = {0x6b558ce3, 0x69e5, 0x4c67, {0xa6, 0x34, 0xf7, 0xfe, 0x72, 0xad, 0xbe, 0x84}}
>    gXenBusProtocolGuid                 = {0x3d3ca290, 0xb9a5, 0x11e3, {0xb7, 0x5d, 0xb8, 0xac, 0x6f, 0x7d, 0x65, 0xe6}}
>    gXenIoProtocolGuid                  = {0x6efac84f, 0x0ab0, 0x4747, {0x81, 0xbe, 0x85, 0x55, 0x62, 0x59, 0x04, 0x49}}
> +  gIoMmuAbsentProtocolGuid            = {0xf8775d50, 0x8abd, 0x4adf, {0x92, 0xac, 0x85, 0x3e, 0x51, 0xf6, 0xc8, 0xdc}}
>  
>  [PcdsFixedAtBuild]
>    gUefiOvmfPkgTokenSpaceGuid.PcdOvmfPeiMemFvBase|0x0|UINT32|0
> 

This patch is identical to its v6 counterpart (modulo the superfluous
line in the v6 commit message, pointed out by Jordan, which you have
removed here, thanks), but you forgot to pick up my R-b from v6:

http://mid.mail-archive.com/f6408c5f-91df-400b-2ef4-823ae6c55d28@redhat.com

So,

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo
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