On 03/08/2013 02:01:46 AM, Jia Hongtao-B38951 wrote:

=20
=20

quoted

-----Original Message-----
From: Wood Scott-B07421
Sent: Friday, March 08, 2013 12:38 AM
To: Jia Hongtao-B38951
Cc: David Laight; Wood Scott-B07421; linuxppc-dev@lists.ozlabs.org;
Stuart Yoder
Subject: Re: [PATCH V4] powerpc/85xx: Add machine check handler to =20

fix

quoted

PCIe erratum on mpc85xx

On 03/07/2013 02:06:05 AM, Jia Hongtao-B38951 wrote:

quoted

Here is the ideas from Scott:
"

quoted

+	if (is_in_pci_mem_space(addr)) {
+		inst =3D *(unsigned int *)regs->nip;

Be careful about taking a fault here.  A simple TLB miss should be
safe given that we shouldn't be accessing PCIe in the middle of
exception code, but what if the mapping has gone away (e.g. a
userspace driver had its code munmap()ed or swapped out)?  What if
permissions allow execute but not read (not sure if Linux will =20

allow

quoted

quoted

this, but the hardware does)?

What if it happened in a KVM guest?  You can't access guest =20

addresses

quoted

quoted

directly.
"

That means you need to be careful about how you read the =20

instruction, not

quoted

that you shouldn't do it at all.

-Scott

=20
I agree.
=20
Do you have a more secure way to get the instruction?
Or what should be done to avoid permission break issue?

probe_kernel_address() should take care of userspace issues.  As for =20
KVM, if you see MSR_GS set, bail out and don't apply the workaround.  =20
Let KVM/QEMU deal with it as it wishes (e.g. reflect to the guest and =20
let its machine check handler do the skipping).  On PR-mode KVM (e.g. =20
on e500v2-based chips) there is no MSR_GS and it just looks like =20
userspace code -- for now just pretend it is user mode.

-Scott=