Re: Question about atomics

[Richard Henderson <richard.henderson@linaro.org>]

On 3/7/22 18:18, Warner Losh wrote:
> I have a question related to the user-mode emulation and atomics. I asked on IRC, but 
> thinking about it, I think it may be too complex to discuss in that medium...
> 
> In FreeBSD we have a system call that uses host atomic operations to interact memory that 
> userland also interacts with using atomic operations.
> 
> In bsd-user we call the kernel with a special flag for dealing with 32-bit processes 
> running on a 64-bit kernel. In this case, we use 32-bit-sized atomics to set variables in 
> the address space of the bsd-user guest. This is used when running armv7 binaries on amd64 
> hosts.
> 
> First question: Is this expected to work? I know I'm a bit vague, so as a followup 
> question: If there's restrictions on this, what might they be? Do some classes of atomic 
> operations work, while others may fail or need additional cooperation? Are there any 
> conformance tests I could compile for FreeBSD/armv7 to test the hypothesis that atomic 
> operations are misbehaving?

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().

There are compile-time restrictions on the set of atomic operations, mostly based on what 
the host supports.  But anything that actually compiles is expected to work (there are a 
set of ifdefs if you need something more than the default).

Beyond that, there is start_exclusive() / end_exclusive() which will stop-the-world and 
make sure that the current thread is the only one running.

> Thanks for any help you might be able to give.

Show the code in question?


r~