Re: [PATCH 23/35] x86/fpu: Add helpers for modifying supervisor xstate

[Edgecombe, Rick P]

On Tue, 2022-02-08 at 09:51 +0100, Thomas Gleixner wrote:
> I like the approach in principle, but you still expose the xstate
> internals via the void pointer. It's just a question of time that
> this
> is type casted and abused in interesting ways.

Thanks for taking a look. I have to say though, these changes are
making me scratch my head a bit. Should we really design around callers
digging into mysterious pointers with magic offsets instead of using
easy helpers full of warnings about pitfalls? It should look odd in a
code review too I would think.

> 
> Something like the below untested (on top of the whole series)
> preserves
> the encapsulation and reduces the code at the call sites.
> 
> 
It loses the ability to know which MSR write actually failed. It also
loses the ability to perform read/write logic under a single
transaction. The latter is not needed for this series, but this snippet
from the IBT series does it:

int ibt_get_clear_wait_endbr(void)
{
	void *xstate;
	u64 msr_val = 0;

	if (!current->thread.shstk.ibt)
		return 0;

	xstate = start_update_xsave_msrs(XFEATURE_CET_USER);
	if (!xsave_rdmsrl(xstate, MSR_IA32_U_CET, &msr_val))
		xsave_wrmsrl(xstate, MSR_IA32_U_CET, msr_val &
~CET_WAIT_ENDBR);
	end_update_xsave_msrs();

	return msr_val & CET_WAIT_ENDBR;
}

I suppose we could just add a new function to do that logic in a single
transaction when the time comes. But inventing data structures to
describe work to be passed off to some executor always seems to break
at the first new requirement. What I usually wanted was a programming
language, and I already had it.

Not to bikeshed though, it will still get the job done.