Re: [PATCH v3] PCI: Align pci memory space base address with page size

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Jun 06, 2023 at 06:06:04PM +0800, bibo, mao wrote:
> Huacai,
> 
> Although I post this patch, I think this should be arch specified
> rather than general problem.  X86 has solved this problem, arm64
> with 64K page size is not popular. However LoongArch has this
> problem, page size is 16K rather than 4K. It is the problem of
> LoongArch system rather than generic issue.

I think this is only related to the page size, not to any
LoongArch-specific details.  If that's the case, I don't think the
change should be arch-specific.

> There is such discussion before:
> https://patchwork.kernel.org/project/linux-pci/patch/22400b8828ad44ddbccb876cc5ca3b11@FE-MBX1012.de.bosch.com/#19319457
> 
> UEFI bios sets pci memory space 4K aligned, however Loongarch kernel rescans the pci
> bus and reassigns pci memory resource. So it it strange like this, here is pci memory info on
>  my 7A2000 board.
> root@user-pc:~# lspci -vvv | grep Region
>         Region 5: Memory at e003526e800 (32-bit, non-prefetchable) [size=1K]
>         Region 0: Memory at e003526ec00 (64-bit, non-prefetchable) [size=1K]

I guess these BARs are from different devices, and the problem you're
pointing out is that both BARs end up in the same 16K page (actually
even the same 4K page):

  (gdb) p/x 0xe003526e800 >> 12
  $1 = 0xe003526e
  (gdb) p/x 0xe003526ec00 >> 12
  $2 = 0xe003526e

I agree that's a potential issue because a user mmap of the first
device also gets access to the BAR of the second device.  But it
doesn't seem to be a *new* or LoongArch-specific issue.

So I *think* the point of this patch is to ensure that BARs of
different devices never share a page.  Why is that suddenly an issue
for LoongArch?  Is it only because you see more sharing with 16K pages
than other arches do with 4K pages?

> 在 2023/6/6 16:45, Bibo Mao 写道:
> > Some PCI devices have only 4K memory space size, it is normal in general
> > machines and aligned with page size. However some architectures which
> > support different page size, default page size on LoongArch is 16K, and
> > ARM64 supports page size varying from 4K to 64K. On machines where larger
> > page size is use, memory space region of two different pci devices may be
> > in one page. It is not safe with mmu protection, also VFIO pci device
> > driver requires base address of pci memory space page aligned, so that it
> > can be memory mapped to qemu user space when it is passed-through to vm.

The minimum memory BAR per spec is 128 bytes (not 4K).  You just
demonstrated that even with 4K pages, different devices can share a
page.

> > It consumes more pci memory resource with page size alignment requirement,
> > it should not be a problem on 64 bit system.
> > 
> > Signed-off-by: Bibo Mao <maobibo@loongson.cn>
> > ---
> >  drivers/pci/setup-res.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/pci/setup-res.c b/drivers/pci/setup-res.c
> > index 967f9a758923..55440ae0128d 100644
> > --- a/drivers/pci/setup-res.c
> > +++ b/drivers/pci/setup-res.c
> > @@ -339,6 +339,14 @@ int pci_assign_resource(struct pci_dev *dev, int resno)
> >  		return -EINVAL;
> >  	}
> >  
> > +#ifdef CONFIG_64BIT
> > +	/*
> > +	 * force minimum page alignment for vfio pci usage
> > +	 * supposing there is enough pci memory resource on 64bit system
> > +	 */
> > +	if (res->flags & IORESOURCE_MEM)
> > +		align = max_t(resource_size_t, PAGE_SIZE, align);
> > +#endif

Why is this under CONFIG_64BIT?  That doesn't have anything to do with
the BAR size.  The only reason I can see is that with CONFIG_64BIT=y,
we *might* have more MMIO space to use for BARs.

> >  	size = resource_size(res);
> >  	ret = _pci_assign_resource(dev, resno, size, align);