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

[Andrew Jones]

On Wed, Jun 15, 2022 at 06:57:06PM +0000, Raghavendra Rao Ananta wrote:
> The selftests, when built with newer versions of clang, is found
> to have over optimized guests' ucall() function, and eliminating
> the stores for uc.cmd (perhaps due to no immediate readers). This
> resulted in the userspace side always reading a value of '0', and
> causing multiple test failures.
> 
> As a result, prevent the compiler from optimizing the stores in
> ucall() with WRITE_ONCE().
> 
> Suggested-by: Ricardo Koller <ricarkol@xxxxxxxxxx>
> Suggested-by: Reiji Watanabe <reijiw@xxxxxxxxxx>
> Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
> ---
>  tools/testing/selftests/kvm/lib/aarch64/ucall.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/aarch64/ucall.c b/tools/testing/selftests/kvm/lib/aarch64/ucall.c
> index e0b0164e9af8..be1d9728c4ce 100644
> --- a/tools/testing/selftests/kvm/lib/aarch64/ucall.c
> +++ b/tools/testing/selftests/kvm/lib/aarch64/ucall.c
> @@ -73,20 +73,19 @@ void ucall_uninit(struct kvm_vm *vm)
>  
>  void ucall(uint64_t cmd, int nargs, ...)
>  {
> -	struct ucall uc = {
> -		.cmd = cmd,
> -	};
> +	struct ucall uc = {};
>  	va_list va;
>  	int i;
>  
> +	WRITE_ONCE(uc.cmd, cmd);
>  	nargs = nargs <= UCALL_MAX_ARGS ? nargs : UCALL_MAX_ARGS;
>  
>  	va_start(va, nargs);
>  	for (i = 0; i < nargs; ++i)
> -		uc.args[i] = va_arg(va, uint64_t);
> +		WRITE_ONCE(uc.args[i], va_arg(va, uint64_t));
>  	va_end(va);
>  
> -	*ucall_exit_mmio_addr = (vm_vaddr_t)&uc;
> +	WRITE_ONCE(*ucall_exit_mmio_addr, (vm_vaddr_t)&uc);
>  }
>  
>  uint64_t get_ucall(struct kvm_vm *vm, uint32_t vcpu_id, struct ucall *uc)
> -- 
> 2.36.1.476.g0c4daa206d-goog
>

Reviewed-by: Andrew Jones <drjones@xxxxxxxxxx>

Thanks,
drew