Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces

[Peter Zijlstra]

On Sun, Sep 27, 2020 at 08:38:03AM -0700, Dave Hansen wrote:
> On 9/27/20 3:51 AM, Greg Kroah-Hartman wrote:
> >> +static inline long acrn_hypercall0(unsigned long hcall_id)
> >> +{
> >> +	register long r8 asm("r8");
> >> +	long result;
> >> +
> >> +	/* Nothing can come between the r8 assignment and the asm: */
> >> +	r8 = hcall_id;
> >> +	asm volatile("vmcall\n\t"
> >> +		     : "=a" (result)
> >> +		     : "r" (r8)
> >> +		     : );
> > What keeps an interrupt from happening between the r8 assignment and the
> > asm: ?
> 
> It's probably better phrased something like: "No other C code can come
> between this r8 assignment and the inline asm".  An interrupt would
> actually be fine in there because interrupts save and restore all
> register state, including r8.
> 
> The problem (mentioned in the changelog) is that gcc does not let you
> place data directly into r8.  But, it does allow you to declare a
> register variable that you can assign to use r8.  There might be a
> problem if a function calls was in between and clobber the register,
> thus the "nothing can come between" comment.
> 
> The comment is really intended to scare away anyone from adding printk()'s.
> 
> More information about these register variables is here:
> 
> > https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
> 
> Any better ideas for comments would be greatly appreciated.  It has 4 or
> 5 copies so I wanted it to be succinct.

This is disguisting.. Segher, does this actually work? Nick, does clang
also support this?