Re: [edk2-devel] [PATCH v2] MdeModulePkg/PciHostBridgeDxe: Add readback after final Cfg-Write

["Joe L" <jlotwo@gmail.com>]

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++ Laszlo and Michael

On Tue, Nov 7, 2023 at 10:54 AM Jose Lopez <jlotwo@gmail.com> wrote:

> ++ CC'd
>
>
> On Mon, Nov 6, 2023 at 6:02 PM Joe Lopez <jlotwo@gmail.com> wrote:
>
>> From: joelopez333 <jlotwo@gmail.com>
>>
>>     REF:https://edk2.groups.io/g/devel/topic/102310377#110456
>>
>>     Problem Report:
>>
>>     On AARCH64, there is no ordering guarantee between configuration
>>     space (ECAM) writes and memory space reads (MMIO). ARM AMBA CHI
>>     only guarantees ordering for reads and writes within a single address
>> region,
>>     however, on some systems MMIO and ECAM may be split into separate
>>     address regions.
>>
>>     A problem may arise when an ECAM write is issued a completion before
>> a subsequent
>>     MMIO read is issued and receives a completion.
>>
>>     For example, a typical PCI software flow is the following:
>>
>>     1. ECAM write to device command register to enable memory space
>>     2. MMIO read from device memory space for which access was enabled
>>     in step 1.
>>
>>     There is no guarantee that step 2. will not begin before the
>> completion of step 1.
>>     on systems where ECAM/MMIO are specified as separate address regions,
>> even
>>     if both spaces have the memory attributes device-nGnRnE.
>>
>>     Fix:
>>
>>     - Add a read after the final PCI Configuration space write
>>       in RootBridgeIoPciAccess.
>>
>>     - When configuration space is strongly ordered, this ensures
>>       that program execution cannot continue until the completion
>>       is received for the previous Cfg-Write, which may have side-effects.
>>
>>     - Risk of reading a "write-only" register and causing a CA which
>> leaves the device
>>       unresponsive. The expectation based on the PCI Base Spec v6.1
>> section 7.4 is that
>>       all PCI Spec-defined registers will be readable, however, there may
>> exist
>>       design-specific registers that fall into this category.
>>
>>     Cc: Leif Lindholm <quic_llindhol@quicinc.com>
>>     Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
>>     Cc: Sami Mujawar <sami.mujawar@arm.com>
>>     Cc: Jian J Wang <jian.j.wang@intel.com>
>>     Cc: Liming Gao <gaoliming@byosoft.com.cn>
>>     Cc: Hao A Wu <hao.a.wu@intel.com>
>>     Cc: Ray Ni <ray.ni@intel.com>
>>     Cc: Pedro Falcato <pedro.falcato@gmail.com>
>>     Cc: Michael Brown <mcb30@ipxe.org>
>>     Signed-off-by: Joe Lopez <jlotwo2@gmail.com>
>> ---
>>  MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c
>> b/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c
>> index 157a0ada80..c2dc2018d6 100644
>> --- a/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c
>> +++ b/MdeModulePkg/Bus/Pci/PciHostBridgeDxe/PciRootBridgeIo.c
>> @@ -1238,6 +1238,14 @@ RootBridgeIoPciAccess (
>>      }
>>    }
>>
>> +  //
>> +  // Perform readback after write to confirm completion was received for
>> the last write
>> +  // before subsequent memory operations can be issued.
>> +  //
>> +  if (!Read) {
>> +    PciSegmentRead8 (Address - InStride);
>> +  }
>> +
>>    return EFI_SUCCESS;
>>  }
>>
>> --
>> 2.25.1
>>
>>


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