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@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); > > > > } > > > > 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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales) _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm