Subject: Re: [PATCH v3 1/3] PCI: Allow ATS to be always on for CXL.cache capable devices

From: Nirmoy Das <[email-masked]>
Date: Sun, 8 Mar 2026 21:53:25 +0100

On 08.03.26 21:49, Nirmoy Das wrote:
>
> On 07.03.26 00:41, Nicolin Chen wrote:
>> Controlled by the IOMMU driver, ATS is usually enabled "on demand" 
>> when a
>> device requests a translation service from its associated IOMMU HW 
>> running
>> on the channel of a given PASID. This is working even when a device 
>> has no
>> translation on its RID (i.e., the RID is IOMMU bypassed).
>>
>> However, certain PCIe devices require non-PASID ATS on their RID even 
>> when
>> the RID is IOMMU bypassed. Call this "always on".
>>
>> For instance, the CXL spec notes in "3.2.5.13 Memory Type on CXL.cache":
>> "To source requests on CXL.cache, devices need to get the Host Physical
>> Address (HPA) from the Host by means of an ATS request on CXL.io."
>>
>> In other words, the CXL.cache capability requires ATS; otherwise, it 
>> can't
>> access host physical memory.
>>
>> Introduce a new pci_ats_always_on() helper for the IOMMU driver to 
>> scan a
>> PCI device and shift ATS policies between "on demand" and "always on".
>>
>> Add the support for CXL.cache devices first. Pre-CXL devices will be 
>> added
>> in quirks.c file.
>>
>> Note that pci_ats_always_on() validates against pci_ats_supported(), 
>> so we
>> ensure that untrusted devices (e.g. external ports) will not be 
>> always on.
>> This maintains the existing ATS security policy regarding potential 
>> side-
>> channel attacks via ATS.
>>
>> Cc: [email-masked]
>> Suggested-by: Vikram Sethi <[email-masked]>
>> Suggested-by: Jason Gunthorpe <[email-masked]>
>> Signed-off-by: Nicolin Chen <[email-masked]>
>
> Tested the series with a Type 2 CXL device.
>
> Tested-by: Nirmoy Das <[email-masked]>
>
> Acked-by: Nirmoy Das <[email-masked]>
Sent with wrong email address
Acked-by: Nirmoy Das <[email-masked]>
>
>> ---
>>   include/linux/pci-ats.h       |  3 +++
>>   include/uapi/linux/pci_regs.h |  1 +
>>   drivers/pci/ats.c             | 42 +++++++++++++++++++++++++++++++++++
>>   3 files changed, 46 insertions(+)
>>
>> diff --git a/include/linux/pci-ats.h b/include/linux/pci-ats.h
>> index 75c6c86cf09dc..d14ba727d38b3 100644
>> --- a/include/linux/pci-ats.h
>> +++ b/include/linux/pci-ats.h
>> @@ -12,6 +12,7 @@ int pci_prepare_ats(struct pci_dev *dev, int ps);
>>   void pci_disable_ats(struct pci_dev *dev);
>>   int pci_ats_queue_depth(struct pci_dev *dev);
>>   int pci_ats_page_aligned(struct pci_dev *dev);
>> +bool pci_ats_always_on(struct pci_dev *dev);
>>   #else /* CONFIG_PCI_ATS */
>>   static inline bool pci_ats_supported(struct pci_dev *d)
>>   { return false; }
>> @@ -24,6 +25,8 @@ static inline int pci_ats_queue_depth(struct 
>> pci_dev *d)
>>   { return -ENODEV; }
>>   static inline int pci_ats_page_aligned(struct pci_dev *dev)
>>   { return 0; }
>> +static inline bool pci_ats_always_on(struct pci_dev *dev)
>> +{ return false; }
>>   #endif /* CONFIG_PCI_ATS */
>>     #ifdef CONFIG_PCI_PRI
>> diff --git a/include/uapi/linux/pci_regs.h 
>> b/include/uapi/linux/pci_regs.h
>> index 14f634ab9350d..6ac45be1008b8 100644
>> --- a/include/uapi/linux/pci_regs.h
>> +++ b/include/uapi/linux/pci_regs.h
>> @@ -1349,6 +1349,7 @@
>>   /* CXL r4.0, 8.1.3: PCIe DVSEC for CXL Device */
>>   #define PCI_DVSEC_CXL_DEVICE                0
>>   #define  PCI_DVSEC_CXL_CAP                0xA
>> +#define   PCI_DVSEC_CXL_CACHE_CAPABLE            _BITUL(0)
>>   #define   PCI_DVSEC_CXL_MEM_CAPABLE            _BITUL(2)
>>   #define   PCI_DVSEC_CXL_HDM_COUNT            __GENMASK(5, 4)
>>   #define  PCI_DVSEC_CXL_CTRL                0xC
>> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
>> index ec6c8dbdc5e9c..cf262eb6e6890 100644
>> --- a/drivers/pci/ats.c
>> +++ b/drivers/pci/ats.c
>> @@ -205,6 +205,48 @@ int pci_ats_page_aligned(struct pci_dev *pdev)
>>       return 0;
>>   }
>>   +/*
>> + * CXL r4.0, sec 3.2.5.13 Memory Type on CXL.cache notes: to source 
>> requests on
>> + * CXL.cache, devices need to get the Host Physical Address (HPA) 
>> from the Host
>> + * by means of an ATS request on CXL.io.
>> + *
>> + * In other world, CXL.cache devices cannot access host physical 
>> memory without
>> + * ATS.
>> + */
>> +static bool pci_cxl_ats_always_on(struct pci_dev *pdev)
>> +{
>> +    u16 cap = 0;
>> +    int offset;
>> +
>> +    offset = pci_find_dvsec_capability(pdev, PCI_VENDOR_ID_CXL,
>> +                       PCI_DVSEC_CXL_DEVICE);
>> +    if (!offset)
>> +        return false;
>> +
>> +    pci_read_config_word(pdev, offset + PCI_DVSEC_CXL_CAP, &cap);
>> +
>> +    return cap & PCI_DVSEC_CXL_CACHE_CAPABLE;
>> +}
>> +
>> +/**
>> + * pci_ats_always_on - Whether the PCI device requires ATS to be 
>> always enabled
>> + * @pdev: the PCI device
>> + *
>> + * Returns true, if the PCI device requires ATS for basic functional 
>> operation.
>> + */
>> +bool pci_ats_always_on(struct pci_dev *pdev)
>> +{
>> +    if (pci_ats_disabled() || !pci_ats_supported(pdev))
>> +        return false;
>> +
>> +    /* A VF inherits its PF's requirement for ATS function */
>> +    if (pdev->is_virtfn)
>> +        pdev = pci_physfn(pdev);
>> +
>> +    return pci_cxl_ats_always_on(pdev);
>> +}
>> +EXPORT_SYMBOL_GPL(pci_ats_always_on);
>> +
>>   #ifdef CONFIG_PCI_PRI
>>   void pci_pri_init(struct pci_dev *pdev)
>>   {
>