Re: [PATCH] LoongArch: Align pci memory base address with page size

[Huacai Chen]

On Fri, Jun 2, 2023 at 3:35 PM bibo, mao <maobibo@xxxxxxxxxxx> wrote:
>
>
>
> 在 2023/6/2 14:55, Huacai Chen 写道:
> > On Fri, Jun 2, 2023 at 2:48 PM bibo, mao <maobibo@xxxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> 在 2023/6/2 12:11, Huacai Chen 写道:
> >>> +cc Bjorn
> >>>
> >>> Hi, Bibo,
> >>>
> >>> On Fri, Jun 2, 2023 at 11:07 AM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
> >>>>
> >>>> LoongArch linux kernel uses 16K page size by default, some pci devices have
> >>>> only 4K memory size, it is normal in general architectures. However memory
> >>>> space of different pci devices will share one physical page address space.
> >>>> This is not safe for mmu protection, also UIO and VFIO requires base
> >>>> address of pci memory space page aligned.
> >>>>
> >>>> This patch adds check with function pcibios_align_resource, and set base
> >>>> address of resource page aligned.
> >>>>
> >>>> Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> >>>> ---
> >>>>  arch/loongarch/pci/pci.c | 23 +++++++++++++++++++++++
> >>>>  1 file changed, 23 insertions(+)
> >>>>
> >>>> diff --git a/arch/loongarch/pci/pci.c b/arch/loongarch/pci/pci.c
> >>>> index 2726639150bc..1380f3672ba2 100644
> >>>> --- a/arch/loongarch/pci/pci.c
> >>>> +++ b/arch/loongarch/pci/pci.c
> >>>> @@ -83,6 +83,29 @@ int pcibios_alloc_irq(struct pci_dev *dev)
> >>>>         return acpi_pci_irq_enable(dev);
> >>>>  }
> >>>>
> >>>> +/*
> >>>> + * memory space size of some pci cards is 4K, it is separated with
> >>>> + * different pages for generic architectures, so that mmu protection can
> >>>> + * work with different pci cards. However page size for LoongArch system
> >>>> + * is 16K, memory space of different pci cards may share the same page
> >>>> + * on LoongArch, it is not safe here.
> >>>> + * Also uio drivers and vfio drivers sugguests that base address of memory
> >>>> + * space should page aligned. This function aligns base address with page size
> >>>> + */
> >>>> +resource_size_t pcibios_align_resource(void *data, const struct resource *res,
> >>>> +               resource_size_t size, resource_size_t align)
> >>>> +{
> >>>> +       resource_size_t start = res->start;
> >>>> +
> >>>> +       if (res->flags & IORESOURCE_MEM) {
> >>>> +               if (align & (PAGE_SIZE - 1)) {
> >>>> +                       align = PAGE_SIZE;
> >>>> +                       start = ALIGN(start, align);
> >>> I don't know whether this patch is really needed, but the logic here
> >>> has some problems.
> >>>
> >>> For example, if PAGE_SIZE=16KB, align=18KB, what should we do? Align
> >>> to 16KB or align to 32KB? IMO it should align to 32KB, but in your
> >>> patch it will align to 16KB.
> >> In general pci device is aligned by size, and its value is a power of 2 in value.
> >> I do not see such devices with 18K alignment requirements.
> > If so, you can simply ignore "align" and use  start = ALIGN(start, PAGE_SIZE);
> >
> >>
> >> By pci local bus spec, there are such lines:
> >>
> >> "Devices are free to consume more address space than required, but decoding down
> >> to a 4 KB space for memory is suggested for devices that need less than that amount. For
> >> instance, a device that has 64 bytes of registers to be mapped into Memory Space may
> >> consume up to 4 KB of address space in order to minimize the number of bits in the address
> >> decoder."
> >>
> >> I cannot  think whether it is necessary simply from judging whether other
> >> architectures have similar code. If so, LoongArch system just  always follows others.
> >> It is actually one problem since LoongArch uses 16K page size.
> > As I know, both MIPS and ARM64 can use non-4K pages, but when I grep
> > pcibios_align_resource in the arch directory, none of them do
> > PAGE_SIZE alignment.
> Here is piece of  code in drivers/vfio/pci/vfio_pci_core.c
>                 /*
>                  * Here we don't handle the case when the BAR is not page
>                  * aligned because we can't expect the BAR will be
>                  * assigned into the same location in a page in guest
>                  * when we passthrough the BAR. And it's hard to access
>                  * this BAR in userspace because we have no way to get
>                  * the BAR's location in a page.
>                  */
> no_mmap:
>                 vdev->bar_mmap_supported[bar] = false;
>
> Do you think it is a issue or not?
May be or may not be, if it should be aligned to PAGE_SIZE, then MIPS
and ARM64 also need this.

>
> You can search function pnv_pci_default_alignment or pcibios_align_resource about
> alpha architecture.
Alpha's pcibios_align_resource() have nothing to do with PAGE_SIZE,
pnv_pci_default_alignment() seems to be the case. But if alignment is
really needed, I think it is better to provide a
pcibios_default_alignment() as powerpc.

Huacai
>
> Regards
> Bibo, mao
>
> >
> > Huacai
> >
> >>
> >> Regards
> >> Bibo, Mao
> >>>
> >>> Huacai
> >>>> +               }
> >>>> +       }
> >>>> +       return start;
> >>>> +}
> >>>> +
> >>>>  static void pci_fixup_vgadev(struct pci_dev *pdev)
> >>>>  {
> >>>>         struct pci_dev *devp = NULL;
> >>>> --
> >>>> 2.27.0
> >>>>
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> >>
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