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

[David Laight <David.Laight@ACULAB.COM>]

From: Andrew Jones
> Sent: 16 June 2022 17:26
> 
> On Thu, Jun 16, 2022 at 03:58:52PM +0000, David Laight wrote:
> > From: Andrew Jones
> > > Sent: 16 June 2022 13:03
> > >
> > > 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@google.com>
> > > > Suggested-by: Reiji Watanabe <reijiw@google.com>
> > > > Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
> > > > ---
> > > >  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);
> > > >  }
> >
> > Am I misreading things again?
> > That function looks like it writes the address of an on-stack
> > item into global data.
> 
> The write to the address that the global points at causes a switch
> from guest to host context. The guest's stack remains intact while
> executing host code and the host can access the uc stack variable
> directly by its address. Take a look at lib/aarch64/ucall.c to see
> all the details.

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.

	David

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