Re: [PATCH ats_vtd v5 20/22] pci: add a pci-level API for ATS

[Minwoo Im]

On 24-07-11 19:00:58, CLEMENT MATHIEU--DRIF wrote:
> 
> 
> On 11/07/2024 10:04, Minwoo Im wrote:
> > Caution: External email. Do not open attachments or click links, unless 
> > this email comes from a known sender and you know the content is safe.
> >
> >
> > On 24-07-10 05:17:42, CLEMENT MATHIEU--DRIF wrote:
> >>
> >> On 09/07/2024 23:17, Minwoo Im wrote:
> >>> Caution: External email. Do not open attachments or click links, unless 
> >>> this email comes from a known sender and you know the content is safe.
> >>>
> >>>
> >>> On 24-07-09 11:58:53, CLEMENT MATHIEU--DRIF wrote:
> >>>> On 09/07/2024 12:15, Minwoo Im wrote:
> >>>>> Caution: External email. Do not open attachments or click links, unless 
> >>>>> this email comes from a known sender and you know the content is safe.
> >>>>>
> >>>>>
> >>>>> On 24-07-02 05:52:45, CLEMENT MATHIEU--DRIF wrote:
> >>>>>> From: Clément Mathieu--Drif <clement.mathieu--drif@eviden.com>
> >>>>>>
> >>>>>> Devices implementing ATS can send translation requests using
> >>>>>> pci_ats_request_translation_pasid.
> >>>>>>
> >>>>>> The invalidation events are sent back to the device using the iommu
> >>>>>> notifier managed with pci_register_iommu_tlb_event_notifier and
> >>>>>> pci_unregister_iommu_tlb_event_notifier
> >>>>>>
> >>>>>> Signed-off-by: Clément Mathieu--Drif <clement.mathieu--drif@eviden.com>
> >>>>>> ---
> >>>>>>     hw/pci/pci.c         | 44 +++++++++++++++++++++++++++++++++++++
> >>>>>>     include/hw/pci/pci.h | 52 
> >>>>>> ++++++++++++++++++++++++++++++++++++++++++++
> >>>>>>     2 files changed, 96 insertions(+)
> >>>>>>
> >>>>>> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> >>>>>> index 7a483dd05d..93b816aff2 100644
> >>>>>> --- a/hw/pci/pci.c
> >>>>>> +++ b/hw/pci/pci.c
> >>>>>> @@ -2833,6 +2833,50 @@ void pci_device_unset_iommu_device(PCIDevice 
> >>>>>> *dev)
> >>>>>>         }
> >>>>>>     }
> >>>>>>
> >>>>>> +ssize_t pci_ats_request_translation_pasid(PCIDevice *dev, uint32_t 
> >>>>>> pasid,
> >>>>>> +                                          bool priv_req, bool 
> >>>>>> exec_req,
> >>>>>> +                                          hwaddr addr, size_t length,
> >>>>>> +                                          bool no_write, 
> >>>>>> IOMMUTLBEntry *result,
> >>>>>> +                                          size_t result_length,
> >>>>>> +                                          uint32_t *err_count)
> >>>>>> +{
> >>>>>> +    assert(result_length);
> >>>>>> +    IOMMUMemoryRegion *iommu_mr = 
> >>>>>> pci_device_iommu_memory_region_pasid(dev,
> >>>>>> +                                                                      
> >>>>>>   pasid);
> >>>>>> +    if (!iommu_mr || !pcie_ats_enabled(dev)) {
> >>>>>> +        return -EPERM;
> >>>>>> +    }
> >>>>>> +    return memory_region_iommu_ats_request_translation(iommu_mr, 
> >>>>>> priv_req,
> >>>>>> +                                                       exec_req, 
> >>>>>> addr, length,
> >>>>>> +                                                       no_write, 
> >>>>>> result,
> >>>>>> +                                                       result_length,
> >>>>>> +                                                       err_count);
> >>>>>> +}
> >>>>> Can we use this function not from the endpoint PCI device, but inside 
> >>>>> of the pci
> >>>>> subsystem (hw/pci/pci.c) to make transparent abstraction for ATS 
> >>>>> request from
> >>>>> PCI endpoint device POV?  I guess it would be better to have PCI 
> >>>>> subsystem to
> >>>>> issue ATS request if pcie_ats_enabled(dev) rather than calling from the 
> >>>>> endpoint
> >>>>> side.
> >>>> Hi,
> >>>>
> >>>> This series aims to bring support for SVM (we are trying to integrate
> >>>> the patches bit by bit).
> >>>>    From a spec point of view, I don't know if it would make sense to
> >>>> implement the SVM logic at the PCI level
> >>>> as it's supposed to be implemented by endpoint devices.
> >>> Understood that this series is targeting the SVM usage.  But ATS feature 
> >>> is
> >>> something general to PCI devices, not only just for SVM, so I guess it 
> >>> would be
> >>> better to have caller to `pci_ats_request_translation_pasid()` in pci 
> >>> subsystem
> >>> like pci_dma_rw() to avoid duplicated implementation in the future for the
> >>> other PCI enpoint devices.
> >> Would we store the ATC directly in the PCI subsytem?
> > Yes, endpoint device (e.g., svm.c) should call pci_* helpers in PCI 
> > subsystem
> > with `PCIDevice *pdev instance` which represents the endpoint device itself.
> > By the instance, we can look up the IOTLB entry from the ATC in the PCI
> > subsystem, not the current caller side.
> >
> >>>> However, we could consider providing a reference/reusable/encapsulated
> >>>> implementation of SVM with a simplified API
> >>>> that would call the pci_* functions under the hood.
> >>> I would prefer that PCI devices which want to request ATS translation has 
> >>> no
> >>> additional implementation for ATS, but only pcie_ats_init().
> >> Hi,
> >>
> >> I think both strategies can coexist.
> >> Keeping control can be interesting for people who use Qemu for hardware
> >> prototyping and who generally want to experiment.
> >> We can keep the current PCI-level API for devices that want to
> >> reimplement the logic themselves
> >> and add a kind of "DMA module"/"ATS+PRI module" that works out of the box.
> > I think we should proivde hybrid mode on this.  One for a `generic` cache
> > policy mode for every PCI endpoint devices which can be controlled in the 
> > PCI
> > subsystem for ATC, the other one is that device-specific cache policy mode
> > which let each device implement their own ATC lookup behaviors to optimize
> > their own caching impact.
> >
> >> That module could be called "struct PciDmaModule" and expose a simple
> >> set of functions like pci_dma_module_init, pci_dma_module_read,
> >> pci_dma_module_write.
> >> I think it's important to keep existing DMA API as is to allow devices
> >> to do both "with ATS" and "without ATS" operations.
> >>
> >> Do you agree with that?
> > Indeed.  Keeping the existing APIs is a good choice, but I would like to 
> > have
> > endpoint devices code much more simpler for the generic usages :)
> That's a good point, we will se what we can do once the current work is 
> integrated.
> Thanks for your comment :)

Do you have a plan to repost this series soon? I would like to apply your
patches and review/test this series.  It looks like you've been through some of
fix patches for VT-d, but I'm curious your further plan for the actual SVM
feature :)

> >
> >>>> Do you have a specific use case in mind?
> >>> ATS/PRI is the actual use case, and it's not that different what you are
> >>> targeting for :)
> >>>
> >>>>    >cmd
> >>>>
> >>>>>