Re: Question about atomics

[Paolo Bonzini <pbonzini@redhat.com>]

On 3/8/22 15:09, Warner Losh wrote:
> 
>     Yes, qatomic_foo is expected to work.  It's what we use across
>     threads, and it is expected to work "in kernel mode", i.e. within cpu_loop().
> 
> Even when the writers are done in the context of system calls to the kernel?

Yes.

That said, for the similar syscall in Linux we just forward it to the 
kernel (and the kernel obviously can only do an atomic---no 
start_exclusive/end_exclusive involved).

> And if the system call does this w/o using
> the start_exclusive/end_exclusive stuff, is that a problem?

If it does it without start_exclusive/end_exclusive they must use 
qatomic_foo().  If it does it with start_exclusive/end_exclusive, they 
can even write a compare-and-exchange as

     old = *(uint64_t *)g2h(cs, addr);
     if (old == oldval)
         *(uint64_t *)g2h(cs, addr) = new;

Paolo