2009-10-23 22:26:29

by Suresh Siddha

Subject: [patch] x86, pat: return EINVAL for pci mmap WC request for !pat_enabled

Thomas Schlichter reported:
> X.org uses libpciaccess which tries to mmap with write combining enabled via
> /sys/bus/pci/devices/*/resource0_wc. Currently, when PAT is not enabled, the
> kernel does fall back to uncached mmap. Then libpciaccess thinks it succeeded
> mapping with write combining enabled and does not set up suited MTRR entries.
> ;-(

Instead of silently mapping pci mmap region as UC minus in the case
of !pat_enabled and wc request, we can return error. Eric Anholt mentioned
that caller (like X) typically follows up with UC minus pci mmap request and
if there is a free mtrr slot, caller will manage adding WC mtrr.

Jesse Barnes says:
> Older versions of libpciaccess will behave better if we do it that way
> (iirc it only allocates an MTRR if the resource_wc file doesn't exist or
> fails to get mapped).

Reported-by: Thomas Schlichter <[email protected]>
Signed-off-by: Thomas Schlichter <[email protected]>
Signed-off-by: Suresh Siddha <[email protected]>
Acked-by: Eric Anholt <[email protected]>
Acked-by: Jesse Barnes <[email protected]>
---

diff --git a/arch/x86/pci/i386.c b/arch/x86/pci/i386.c
index b22d13b..a672f12 100644
--- a/arch/x86/pci/i386.c
+++ b/arch/x86/pci/i386.c
@@ -282,6 +282,15 @@ int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma,
return -EINVAL;

prot = pgprot_val(vma->vm_page_prot);
+
+ /*
+ * Return error if pat is not enabled and write_combine is requested.
+ * Caller can followup with UC MINUS request and add a WC mtrr if there
+ * is a free mtrr slot.
+ */
+ if (!pat_enabled && write_combine)
+ return -EINVAL;
+
if (pat_enabled && write_combine)
prot |= _PAGE_CACHE_WC;
else if (pat_enabled || boot_cpu_data.x86 > 3)

2009-10-26 21:12:51

by Jesse Barnes

Subject: Re: [patch] x86, pat: return EINVAL for pci mmap WC request for !pat_enabled

On Mon, 26 Oct 2009 13:21:32 -0800
Suresh Siddha <[email protected]> wrote:

> Thomas Schlichter reported:
> > X.org uses libpciaccess which tries to mmap with write combining
> > enabled via /sys/bus/pci/devices/*/resource0_wc. Currently, when
> > PAT is not enabled, the kernel does fall back to uncached mmap.
> > Then libpciaccess thinks it succeeded mapping with write combining
> > enabled and does not set up suited MTRR entries. ;-(
>
> Instead of silently mapping pci mmap region as UC minus in the case
> of !pat_enabled and wc request, we can return error. Eric Anholt
> mentioned that caller (like X) typically follows up with UC minus pci
> mmap request and if there is a free mtrr slot, caller will manage
> adding WC mtrr.
>
> Jesse Barnes says:
> > Older versions of libpciaccess will behave better if we do it that
> > way (iirc it only allocates an MTRR if the resource_wc file doesn't
> > exist or fails to get mapped).
>
> Reported-by: Thomas Schlichter <[email protected]>
> Signed-off-by: Thomas Schlichter <[email protected]>
> Signed-off-by: Suresh Siddha <[email protected]>
> Acked-by: Eric Anholt <[email protected]>
> Acked-by: Jesse Barnes <[email protected]>
> ---
>

Applied to my linux-next branch, thanks.

--
Jesse Barnes, Intel Open Source Technology Center