Re: [PATCH v1 1/1] x86/fred: Fix the FRED RSP0 MSR out of sync with its per CPU cache

[Dave Hansen <dave.hansen@intel.com>]

On 1/7/25 18:36, Xin Li (Intel) wrote:
> The FRED RSP0 MSR (pointing to the top of the kernel stack for user
> level event delivery) and its per CPU cache should be kept in sync to
> avoid redundant writes in the exit to user space path, as a result,
> a write to the FRED RSP0 MSR is paired with a write to its per CPU
> cache as fred_update_rsp0() does.

I _think_ you're trying to explain the general use of a per-cpu MSR
cache. That's good. But I was reading this paragraph and thinking at
this point that the bug had something to do with redundant writes to the
MSR.

How about this?

	The FRED RSP0 MSR is only used for delivering events when
	running userspace. This kernel leverages this property to reduce
	expensive MSR writes and optimize context switches.  The kernel
	only writes the MSR when about to run userspace *and* when the
	MSR has actually changed since the last time userspace ran.

	This optimization is implemented by maintaining a per-cpu cache
	of FRED RSP0 and then checking that against the value for the
	current task's stack before running userspace.

	However cpu_init_fred_exceptions() writes the MSR without
	updating the per-cpu cache. This means that the kernel might
	return to userspace with MSR_IA32_FRED_RSP0==0 when it needed to
	point to the current task stack. This would induce a double
	fault (#DF), which is bad.

	A context switch after cpu_init_fred_exceptions() can paper over
	the issue since it updates the cached value. That evidently
	happens most of the time explaining how this bug got through.