On 25/09/19 15:12, Vitaly Kuznetsov wrote:
> After commit e8bb4755eea2("KVM: selftests: Split ucall.c into architecture
> specific files") selftests which use ucall on x86 started segfaulting and
> apparently it's gcc to blame: it "optimizes" ucall() function throwing away
> va_start/va_end part because it thinks the structure is not being used.
> Previously, it couldn't do that because the there was also MMIO version and
> the decision which particular implementation to use was done at runtime.
>
> With older gccs it's possible to solve the problem by adding 'volatile'
> to 'struct ucall' but at least with gcc-8.3 this trick doesn't work.
>
> 'memory' clobber seems to do the job.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
> s390 should, in theory, have the same problem. Thomas, Cornelia, could
> you please take a look? Thanks!
> ---
> tools/testing/selftests/kvm/lib/x86_64/ucall.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/kvm/lib/x86_64/ucall.c b/tools/testing/selftests/kvm/lib/x86_64/ucall.c
> index 4bfc9a90b1de..da4d89ad5419 100644
> --- a/tools/testing/selftests/kvm/lib/x86_64/ucall.c
> +++ b/tools/testing/selftests/kvm/lib/x86_64/ucall.c
> @@ -32,7 +32,7 @@ void ucall(uint64_t cmd, int nargs, ...)
> va_end(va);
>
> asm volatile("in %[port], %%al"
> - : : [port] "d" (UCALL_PIO_PORT), "D" (&uc) : "rax");
> + : : [port] "d" (UCALL_PIO_PORT), "D" (&uc) : "rax", "memory");
> }
>
> uint64_t get_ucall(struct kvm_vm *vm, uint32_t vcpu_id, struct ucall *uc)
>
Queued, thanks. s390 already clobbers memory.
Paolo
