-----Original Message-----
From: Wood Scott-B07421
Sent: Saturday, March 09, 2013 8:49 AM
To: Jia Hongtao-B38951
Cc: Wood Scott-B07421; David Laight; linuxppc-dev@lists.ozlabs.org;
Stuart Yoder
Subject: Re: [PATCH V4] powerpc/85xx: Add machine check handler to fix
PCIe erratum on mpc85xx
=20
On 03/08/2013 02:01:46 AM, Jia Hongtao-B38951 wrote:

quoted

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

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

allow

quoted

quoted

this, but the hardware does)?

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

addresses

quoted

quoted

directly.
"

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

instruction, not

quoted

that you shouldn't do it at all.

-Scott

I agree.

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

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

Hi Scott,

Is that OK if I use the following code?

	u32 inst;
	int ret;

	if (is_in_pci_mem_space(addr)) {
		if (!user_mode(regs)) {
			ret =3D probe_kernel_address(regs->nip, inst);

			if (!ret) {
				rd =3D get_rt(inst);
				regs->gpr[rd] =3D 0xffffffff;
			}
		}

		regs->nip +=3D 4;
		return 1;
	}


Thanks.
-Hongtao.