linux-kernel - Re: [PATCH] ACPI: VIOT: Fix ACS setup

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Date:   Wed, 29 Jun 2022 21:15:19 +0200
From:   Eric Auger <eric.auger.pro@...il.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Eric Auger <eric.auger@...hat.com>
Cc:     jroedel@...e.de, linux-acpi@...r.kernel.org,
        iommu@...ts.linux-foundation.org, rafael@...nel.org,
        lenb@...nel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux.dev
Subject: Re: [PATCH] ACPI: VIOT: Fix ACS setup

Hi Jean

On 6/29/22 11:14, Jean-Philippe Brucker wrote:
> Hi Eric,
> 
> On Mon, Jun 27, 2022 at 02:55:34PM +0200, Eric Auger wrote:
>> Currently acpi_viot_init() gets called after the pci
>> device has been scanned and pci_enable_acs() has been called.
>> So pci_request_acs() fails to be taken into account leading
>> to wrong single iommu group topologies when dealing with
>> multi-function root ports for instance.
>>
>> We cannot simply move the acpi_viot_init() earlier, similarly
>> as the IORT init because the VIOT parsing relies on the pci
>> scan. However we can detect VIOT is present earlier and in
>> such a case, request ACS. Introduce a new acpi_viot_early_init()
>> routine that allows to call pci_request_acs() before the scan.
>>
>> Fixes: 3cf485540e7b ("ACPI: Add driver for the VIOT table")
>> Signed-off-by: Eric Auger <eric.auger@...hat.com>
>> Reported-by: Jin Liu <jinl@...hat.com>
> 
> Thanks for the fix, the patch makes sense and fixes the issue.
> 
> I wondered whether we should keep the logic where we only request ACS if
> an IOMMU is found to manage a PCI range, but I can't see any harm in
> requesting it regardless (plus there is a precedent with AMD IOMMU).
Yes that's what I saw too
> I could imagine some VMM wanting to only put an IOMMU in front of its MMIO
> devices and leave PCI to roam free, but that seems like a stretch.
> 
> There is another issue with the existing code, though: we can't call
> pci_request_acs() when CONFIG_PCI is disabled because no stub is defined.
> Could you wrap the call in an #ifdef?
sure
> 
>> ---
>>  drivers/acpi/bus.c        |  1 +
>>  drivers/acpi/viot.c       | 23 +++++++++++++++++------
>>  include/linux/acpi_viot.h |  2 ++
>>  3 files changed, 20 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
>> index 86fa61a21826..906ad8153fd9 100644
>> --- a/drivers/acpi/bus.c
>> +++ b/drivers/acpi/bus.c
>> @@ -1400,6 +1400,7 @@ static int __init acpi_init(void)
>>  
>>  	pci_mmcfg_late_init();
>>  	acpi_iort_init();
>> +	acpi_viot_early_init();
>>  	acpi_hest_init();
>>  	acpi_ghes_init();
>>  	acpi_scan_init();
>> diff --git a/drivers/acpi/viot.c b/drivers/acpi/viot.c
>> index d2256326c73a..3c1be123e4d6 100644
>> --- a/drivers/acpi/viot.c
>> +++ b/drivers/acpi/viot.c
>> @@ -248,6 +248,23 @@ static int __init viot_parse_node(const struct acpi_viot_header *hdr)
>>  	return ret;
>>  }
>>  
>> +/**
>> + * acpi_viot_early_init - Test the presence of VIOT and enable ACS
>> + *
>> + * If the VIOT does exist, ACS must be enabled. This cannot be
>> + * done in acpi_viot_init() which is called after the bus scan
>> + */
>> +void __init acpi_viot_early_init(void)
>> +{
>> +	acpi_status status;
>> +	struct acpi_table_header *hdr;
>> +
>> +	status = acpi_get_table(ACPI_SIG_VIOT, 0, &hdr);
>> +	if (!ACPI_FAILURE(status))
>> +		pci_request_acs();
>> +	acpi_put_table(hdr);
> 
> I'd rather not call acpi_put_table() in case of failure. I know it is
> handled but it looks fragile and I couldn't find any other user of
> acpi_get_table() doing this.
OK
> 
>> +}
>> +
>>  /**
>>   * acpi_viot_init - Parse the VIOT table
>>   *
>> @@ -319,12 +336,6 @@ static int viot_pci_dev_iommu_init(struct pci_dev *pdev, u16 dev_id, void *data)
>>  			epid = ((domain_nr - ep->segment_start) << 16) +
>>  				dev_id - ep->bdf_start + ep->endpoint_id;
>>  
>> -			/*
>> -			 * If we found a PCI range managed by the viommu, we're
>> -			 * the one that has to request ACS.
>> -			 */
>> -			pci_request_acs();
>> -
>>  			return viot_dev_iommu_init(&pdev->dev, ep->viommu,
>>  						   epid);
>>  		}
>> diff --git a/include/linux/acpi_viot.h b/include/linux/acpi_viot.h
>> index 1eb8ee5b0e5f..e58d60f8ff2e 100644
>> --- a/include/linux/acpi_viot.h
>> +++ b/include/linux/acpi_viot.h
>> @@ -6,10 +6,12 @@
>>  #include <linux/acpi.h>
>>  
>>  #ifdef CONFIG_ACPI_VIOT
>> +void __init acpi_viot_early_init(void);
>>  void __init acpi_viot_init(void);
>>  int viot_iommu_configure(struct device *dev);
>>  #else
>>  static inline void acpi_viot_init(void) {}
>> +static inline void acpi_viot_early_init(void) {}
> 
> nit: different declaration order
OK

Thanks

Eric
> 
> Thanks,
> Jean
> 
> 
>>  static inline int viot_iommu_configure(struct device *dev)
>>  {
>>  	return -ENODEV;
>> -- 
>> 2.35.3
>>