On Thu, Dec 03, 2020 at 07:25:43PM +0800, Rongwei Wang wrote:

quoted

2020年12月3日 下午4:35，Marc Zyngier [off-list ref] 写道：

On 2020-12-03 05:45, Rongwei Wang wrote:

quoted

quoted

2020年12月1日 下午11:37，Marc Zyngier [off-list ref] 写道：
On 2020-12-01 14:25, wangrongwei wrote:

quoted

quoted

2020年12月1日 下午4:12，Marc Zyngier [off-list ref] 写道：
On 2020-12-01 03:09, wangrongwei wrote:

quoted

Hi
We have validate this driver in vm and physical machine, and works fine.

But what does "work fine" mean? None of these system registers are supposed
to be accessible from userspace, so please explain *what* you are trying to
do with this, other that introducing security holes and general system
instability?

I think I know what you mean. Do you want me to describe how we achieved it?
In x86, the different registers can be accessed directly using the
rdmsr and wrmsr instructions, but in ARM, since these two instructions
are missing, so we modify the code segment during runtime, similar to
the principle of static_key.

[...]
These are implementation details, none of which answer my question:
What makes you think this is a good idea? I cannot see any legitimate

In fact, I think this tool useful mainly in the following scenarios:
	1. performance debug
	2. Arm-core features test
	3. Debug-tool for kernel developer
Also, for example, MSR-ARM is needed for chip verification and
system-level functional verification.
A simple example, perf stat can test pmu, but the overflow interrupt
function and forced overflow function of pmu is not covered.

But what does it mean to change random system registers while the kernel
itself is using them in parallel? All you are introducing is a bunch of
uncontrolled, unexpected, and possibly fatal side effects.

This problem exists when writing to a register, but it does not exist when reading a register.

Reading registers can also have side-effects. For example the ICC_IAR_*
registers are read-sensitive, and reading those could cause the kernel
to stop receiving timer interrupts or similar. The problem /does/ exist
when reading registers.

I concur with Marc and others that this simply isn't safe, regardless of
read or write, and not something we want upstream in Linux.

For debug and testing, I suggest looking at kvm-unit-tests. That has
some PMU tests already, and is gaining the ability to run bare-metal
around now (and so might be helpful for testing HW). Having improved
tests there would benefit everyone.

Thanks,
Mark.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel