Re: [PATCH] selftests: KVM: Handle compiler optimizations in ucall

[Paolo Bonzini <pbonzini@redhat.com>]

On 6/17/22 09:28, Andrew Jones wrote:
> On Thu, Jun 16, 2022 at 09:54:16PM +0000, David Laight wrote:
>> From: oliver.upton@linux.dev
>>> Sent: 16 June 2022 19:45
>>
>>>
>>> June 16, 2022 11:48 AM, "David Laight" <David.Laight@aculab.com> wrote:
>>>> No wonder I was confused.
>>>> It's not surprising the compiler optimises it all away.
>>>>
>>>> It doesn't seem right to be 'abusing' WRITE_ONCE() here.
>>>> Just adding barrier() should be enough and much more descriptive.
>>>
>>> I had the same thought, although I do not believe barrier() is sufficient
>>> on its own. barrier_data() with a pointer to uc passed through
>>> is required to keep clang from eliminating the dead store.
>>
>> A barrier() (full memory clobber) ought to be stronger than
>> the partial one than barrier_data() generates.
>>
>> I can't quite decide whether you need a barrier() both sides
>> of the 'magic write'.
>> Plausibly the compiler could discard the on-stack data
>> after the barrier() and before the 'magic write'.
>>
>> Certainly putting the 'magic write' inside a asm block
>> that has a memory clobber is a more correct solution.
> 
> Indeed, since the magic write is actually a guest MMIO write, then
> it should be using writeq().

It doesn't need to use writeq() because no special precautions are 
needed with respect to cacheability or instruction reordering (as is the 
case with hardware registers).

WRITE_ONCE is okay, especially since the code never reads it (and if it 
did it would also use READ_ONCE).

Paolo