Re: [PATCH v7 2/2] PCI: dwc: Fix MSI page leakage in suspend/resume

[Robin Murphy <robin.murphy@arm.com>]

On 2020-10-14 17:52, Ard Biesheuvel wrote:
> On Mon, 12 Oct 2020 at 13:38, Robin Murphy <robin.murphy@arm.com> wrote:
>>
>> On 2020-10-09 08:55, Jisheng Zhang wrote:
>>> Currently, dw_pcie_msi_init() allocates and maps page for msi, then
>>> program the PCIE_MSI_ADDR_LO and PCIE_MSI_ADDR_HI. The Root Complex
>>> may lose power during suspend-to-RAM, so when we resume, we want to
>>> redo the latter but not the former. If designware based driver (for
>>> example, pcie-tegra194.c) calls dw_pcie_msi_init() in resume path, the
>>> msi page will be leaked.
>>>
>>> As pointed out by Rob and Ard, there's no need to allocate a page for
>>> the MSI address, we could use an address in the driver data.
>>>
>>> To avoid map the MSI msg again during resume, we move the map MSI msg
>>> from dw_pcie_msi_init() to dw_pcie_host_init().
>>
>> You should move the unmap there as well. As soon as you know what the
>> relevant address would be if you *were* to do DMA to this location, then
>> the exercise is complete. Leaving it mapped for the lifetime of the
>> device in order to do not-DMA to it seems questionable (and represents
>> technically incorrect API usage without at least a sync_for_cpu call
>> before any other access to the data).
>>
>> Another point of note is that using streaming DMA mappings at all is a
>> bit fragile (regardless of this change). If the host controller itself
>> has a limited DMA mask relative to physical memory (which integrators
>> still seem to keep doing...) then you could end up punching your MSI
>> hole right in the middle of the SWIOTLB bounce buffer, where it's then
>> almost *guaranteed* to interfere with real DMA :(
>>
> 
> Wouldn't it be the unmap you are suggesting that would create this
> problem? If the bounce buffer is never released, the fake MSI doorbell
> address can never conflict with any other DMA mappings.

True, it's just that hogging a SWIOTLB slot for effectively the entire 
lifetime of the system is such an anathema that it didn't even cross my 
mind...

Ugh, so I guess the patch as it stands does actually work out more 
robust than I gave it credit for, even if the reason is horrid... oh well :)

Robin.

>> If no DWC users have that problem and the current code is working well
>> enough, then I see little reason not to make this partucular change to
>> tidy up the implementation, just bear in mind that there's always the
>> possibility of having to come back and change it yet again in future to
>> make it more robust. I had it in mind that this trick was done with a
>> coherent DMA allocation, which would be safe from addressing problems
>> but would need to be kept around for the lifetime of the device, but
>> maybe that was a different driver :/
>>
>> Robin.
>>
>>> Suggested-by: Rob Herring <robh@kernel.org>
>>> Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
>>> Reviewed-by: Rob Herring <robh@kernel.org>
>>> ---
>>>    drivers/pci/controller/dwc/pci-dra7xx.c       | 18 +++++++++-
>>>    .../pci/controller/dwc/pcie-designware-host.c | 33 ++++++++++---------
>>>    drivers/pci/controller/dwc/pcie-designware.h  |  2 +-
>>>    3 files changed, 36 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/drivers/pci/controller/dwc/pci-dra7xx.c b/drivers/pci/controller/dwc/pci-dra7xx.c
>>> index 8f0b6d644e4b..6d012d2b1e90 100644
>>> --- a/drivers/pci/controller/dwc/pci-dra7xx.c
>>> +++ b/drivers/pci/controller/dwc/pci-dra7xx.c
>>> @@ -466,7 +466,9 @@ static struct irq_chip dra7xx_pci_msi_bottom_irq_chip = {
>>>    static int dra7xx_pcie_msi_host_init(struct pcie_port *pp)
>>>    {
>>>        struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
>>> +     struct device *dev = pci->dev;
>>>        u32 ctrl, num_ctrls;
>>> +     int ret;
>>>
>>>        pp->msi_irq_chip = &dra7xx_pci_msi_bottom_irq_chip;
>>>
>>> @@ -482,7 +484,21 @@ static int dra7xx_pcie_msi_host_init(struct pcie_port *pp)
>>>                                    ~0);
>>>        }
>>>
>>> -     return dw_pcie_allocate_domains(pp);
>>> +     ret = dw_pcie_allocate_domains(pp);
>>> +     if (ret)
>>> +             return ret;
>>> +
>>> +     pp->msi_data = dma_map_single_attrs(dev, &pp->msi_msg,
>>> +                                        sizeof(pp->msi_msg),
>>> +                                        DMA_FROM_DEVICE,
>>> +                                        DMA_ATTR_SKIP_CPU_SYNC);
>>> +     ret = dma_mapping_error(dev, pp->msi_data);
>>> +     if (ret) {
>>> +             dev_err(dev, "Failed to map MSI data\n");
>>> +             pp->msi_data = 0;
>>> +             dw_pcie_free_msi(pp);
>>> +     }
>>> +     return ret;
>>>    }
>>>
>>>    static const struct dw_pcie_host_ops dra7xx_pcie_host_ops = {
>>> diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
>>> index d3e9ea11ce9e..d02c7e74738d 100644
>>> --- a/drivers/pci/controller/dwc/pcie-designware-host.c
>>> +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
>>> @@ -266,30 +266,23 @@ void dw_pcie_free_msi(struct pcie_port *pp)
>>>        irq_domain_remove(pp->msi_domain);
>>>        irq_domain_remove(pp->irq_domain);
>>>
>>> -     if (pp->msi_page)
>>> -             __free_page(pp->msi_page);
>>> +     if (pp->msi_data) {
>>> +             struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
>>> +             struct device *dev = pci->dev;
>>> +
>>> +             dma_unmap_single_attrs(dev, pp->msi_data, sizeof(pp->msi_msg),
>>> +                                    DMA_FROM_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);
>>> +     }
>>>    }
>>>
>>>    void dw_pcie_msi_init(struct pcie_port *pp)
>>>    {
>>>        struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
>>> -     struct device *dev = pci->dev;
>>> -     u64 msi_target;
>>> +     u64 msi_target = (u64)pp->msi_data;
>>>
>>>        if (!IS_ENABLED(CONFIG_PCI_MSI))
>>>                return;
>>>
>>> -     pp->msi_page = alloc_page(GFP_KERNEL);
>>> -     pp->msi_data = dma_map_page(dev, pp->msi_page, 0, PAGE_SIZE,
>>> -                                 DMA_FROM_DEVICE);
>>> -     if (dma_mapping_error(dev, pp->msi_data)) {
>>> -             dev_err(dev, "Failed to map MSI data\n");
>>> -             __free_page(pp->msi_page);
>>> -             pp->msi_page = NULL;
>>> -             return;
>>> -     }
>>> -     msi_target = (u64)pp->msi_data;
>>> -
>>>        /* Program the msi_data */
>>>        dw_pcie_writel_dbi(pci, PCIE_MSI_ADDR_LO, lower_32_bits(msi_target));
>>>        dw_pcie_writel_dbi(pci, PCIE_MSI_ADDR_HI, upper_32_bits(msi_target));
>>> @@ -394,6 +387,16 @@ int dw_pcie_host_init(struct pcie_port *pp)
>>>                                irq_set_chained_handler_and_data(pp->msi_irq,
>>>                                                            dw_chained_msi_isr,
>>>                                                            pp);
>>> +
>>> +                     pp->msi_data = dma_map_single_attrs(pci->dev, &pp->msi_msg,
>>> +                                                   sizeof(pp->msi_msg),
>>> +                                                   DMA_FROM_DEVICE,
>>> +                                                   DMA_ATTR_SKIP_CPU_SYNC);
>>> +                     if (dma_mapping_error(pci->dev, pp->msi_data)) {
>>> +                             dev_err(pci->dev, "Failed to map MSI data\n");
>>> +                             pp->msi_data = 0;
>>> +                             goto err_free_msi;
>>> +                     }
>>>                } else {
>>>                        ret = pp->ops->msi_host_init(pp);
>>>                        if (ret < 0)
>>> diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
>>> index 97c7063b9e89..9d2f511f13fa 100644
>>> --- a/drivers/pci/controller/dwc/pcie-designware.h
>>> +++ b/drivers/pci/controller/dwc/pcie-designware.h
>>> @@ -190,8 +190,8 @@ struct pcie_port {
>>>        int                     msi_irq;
>>>        struct irq_domain       *irq_domain;
>>>        struct irq_domain       *msi_domain;
>>> +     u16                     msi_msg;
>>>        dma_addr_t              msi_data;
>>> -     struct page             *msi_page;
>>>        struct irq_chip         *msi_irq_chip;
>>>        u32                     num_vectors;
>>>        u32                     irq_mask[MAX_MSI_CTRLS];
>>>
>>
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