Re: [PATCH v4 16/27] hw/pci-host/gpex: Allow to generate preserve boot config DSM #5

[Eric Auger <eric.auger@redhat.com>]

On 10/1/25 3:05 PM, Jonathan Cameron wrote:
> On Mon, 29 Sep 2025 14:36:32 +0100
> Shameer Kolothum <skolothumtho@nvidia.com> wrote:
>
>> From: Eric Auger <eric.auger@redhat.com>
>>
>> Add a 'preserve_config' field in struct GPEXConfig and if set, generate the
>> DSM #5 for preserving PCI boot configurations. For SMMUV3 accel=on support,
>> we are making use of IORT RMRs in a subsequent patch and that requires the
>> DSM #5.
>>
>> At the moment the DSM generation is not yet enabled.
>>
>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>> [Shameer: Removed possible duplicate _DSM creations]
>> Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
>> Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
> Throw an AML blob in the patch description as easier to check that against
> the spec.  Add a specific spec reference as well.
>
>> ---
>> Previously, QEMU reverted an attempt to enable DSM #5 because it caused a
>> regression,
>> https://lore.kernel.org/all/20210724185234.GA2265457@roeck-us.net/.
>>
>> However, in this series, we enable it selectively, only when SMMUv3 is in
>> accelerator mode. The devices involved in the earlier regression are not
>> expected in accelerated SMMUv3 use cases.
>> ---
>>  hw/pci-host/gpex-acpi.c    | 29 +++++++++++++++++++++++------
>>  include/hw/pci-host/gpex.h |  1 +
>>  2 files changed, 24 insertions(+), 6 deletions(-)
>>
>> diff --git a/hw/pci-host/gpex-acpi.c b/hw/pci-host/gpex-acpi.c
>> index 4587baeb78..e3825ed0b1 100644
>> --- a/hw/pci-host/gpex-acpi.c
>> +++ b/hw/pci-host/gpex-acpi.c
>> @@ -51,10 +51,11 @@ static void acpi_dsdt_add_pci_route_table(Aml *dev, uint32_t irq,
>>      }
>>  }
>>  
>> -static Aml *build_pci_host_bridge_dsm_method(void)
>> +static Aml *build_pci_host_bridge_dsm_method(bool preserve_config)
>>  {
>>      Aml *method = aml_method("_DSM", 4, AML_NOTSERIALIZED);
>>      Aml *UUID, *ifctx, *ifctx1, *buf;
>> +    uint8_t byte_list[1] = {0};
> The inline declaration is a bit odd, but I'm not seeing a specific reason to
> change that here.  Perhaps call out the change as some 'other cleanup' in the
> patch description if you want to make it anyway.
I think this is a good cleanup anyway.
>
>>  
>>      /* PCI Firmware Specification 3.0
>>       * 4.6.1. _DSM for PCI Express Slot Information
>> @@ -64,10 +65,23 @@ static Aml *build_pci_host_bridge_dsm_method(void)
>>      UUID = aml_touuid("E5C937D0-3553-4D7A-9117-EA4D19C3434D");
>>      ifctx = aml_if(aml_equal(aml_arg(0), UUID));
>>      ifctx1 = aml_if(aml_equal(aml_arg(2), aml_int(0)));
>> -    uint8_t byte_list[1] = {0};
>> +    if (preserve_config) {
>> +        /* support for function 0 and function 5 */
>> +        byte_list[0] = 0x21;
> Change the comment to reflect the fix in previous patch as otherwise
> it sounds like bit(0) means function 0 is supported.
>
>        /* support functions other than 0, specifically function 5 */

yeah I was confused too after all this time. Maybe quote the ACPI spec
at least in the commit msg:

"
If Function Index is zero, the return is a buffer containing one bit for
each function index, starting with zero. Bit 0 indicates whether there
is support for any functions other than function 0 for the specified
UUID and Revision ID. If set to zero, no functions are supported (other
than function zero) for the specified UUID and Revision ID. If set to
one, at least one additional function is supported. For all other bits
in the buffer, a bit is set to zero to indicate if that function index
is not supported for the specific UUID and Revision ID. (For example,
bit 1 set to 0 indicates that function index 1 is not supported for the
specific UUID and Revision ID.)
"

Eric


>
>> +    }
>>      buf = aml_buffer(1, byte_list);
>>      aml_append(ifctx1, aml_return(buf));
>>      aml_append(ifctx, ifctx1);
>> +    if (preserve_config) {
>> +        Aml *ifctx2 = aml_if(aml_equal(aml_arg(2), aml_int(5)));
>> +        /*
>> +         * 0 - The operating system must not ignore the PCI configuration that
>> +         *     firmware has done at boot time.
>> +         */
>> +        aml_append(ifctx2, aml_return(aml_int(0)));
>> +        aml_append(ifctx, ifctx2);
>> +    }
>> +
>>      aml_append(method, ifctx);
>>  
>>      byte_list[0] = 0;
>> @@ -77,12 +91,13 @@ static Aml *build_pci_host_bridge_dsm_method(void)
>>  }