Re: [RFC 1/1] KVM: selftests: rseq_test: use vdso_getcpu() instead of syscall()

Robert Hoo
2022-11-02T12:46:59Z

On Wed, 2022-11-02 at 12:24 +0800, Gavin Shan wrote:
> Hi Robert,
>
> On 11/2/22 10:01 AM, Robert Hoo wrote:
> > vDSO getcpu() has been in Kernel since 2.6.19, which we can assume
> > generally available.
> > Use vDSO getcpu() to reduce the overhead, so that vcpu thread
> > stalls less
> > therefore can have more odds to hit the race condition.
> >
>
> It would be nice to provide more context to explain how the race
> condition is caused.

OK. How about this?
... hit the race condition that vcpu_run() inside need to handle pcpu
migration triggered by sched_setaffinity() in migration thread.
>
> > Fixes: 0fcc102923de ("KVM: selftests: Use getcpu() instead of
> > sched_getcpu() in rseq_test")
> > Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
> > ---
> > tools/testing/selftests/kvm/rseq_test.c | 32 ++++++++++++++++++
> > -------
> > 1 file changed, 24 insertions(+), 8 deletions(-)
> >
> > diff --git a/tools/testing/selftests/kvm/rseq_test.c
> > b/tools/testing/selftests/kvm/rseq_test.c
> > index 6f88da7e60be..0b68a6b19b31 100644
> > --- a/tools/testing/selftests/kvm/rseq_test.c
> > +++ b/tools/testing/selftests/kvm/rseq_test.c
> > @@ -42,15 +42,29 @@ static void guest_code(void)
> > }
> >
> > /*
> > - * We have to perform direct system call for getcpu() because it's
> > - * not available until glic 2.29.
> > + * getcpu() was added in kernel 2.6.19. glibc support wasn't there
> > + * until glibc 2.29.
> > + * We can direct call it from vdso to ease gblic dependency.
> > + *
> > + * vdso manipulation code refers from
> > selftests/x86/test_vsyscall.c
> > */
> > -static void sys_getcpu(unsigned *cpu)
> > -{
> > - int r;
> > +typedef long (*getcpu_t)(unsigned *, unsigned *, void *);
> > +static getcpu_t vdso_getcpu;
> >
> > - r = syscall(__NR_getcpu, cpu, NULL, NULL);
> > - TEST_ASSERT(!r, "getcpu failed, errno = %d (%s)", errno,
> > strerror(errno));
> > +static void init_vdso(void)
> > +{
> > + void *vdso = dlopen("linux-vdso.so.1", RTLD_LAZY | RTLD_LOCAL |
> > + RTLD_NOLOAD);
> > + if (!vdso)
> > + vdso = dlopen("linux-gate.so.1", RTLD_LAZY | RTLD_LOCAL
> > |
> > + RTLD_NOLOAD);
> > + if (!vdso)
> > + TEST_ASSERT(!vdso, "failed to find vDSO\n");
> > +
> > + vdso_getcpu = (getcpu_t)dlsym(vdso, "__vdso_getcpu");
> > + if (!vdso_getcpu)
> > + TEST_ASSERT(!vdso_getcpu,
> > + "failed to find __vdso_getcpu in vDSO\n");
> > }
> >
>
> As the comments say, vdso manipulation code comes from
> selftests/x86/test_vsyscall.c.
> I would guess 'linux-vdso.so.1' and 'linux-gate.so.1' are x86
> specific. If I'm correct,
> the test case will fail on other architectures, including ARM64.
>
Ah, right, thanks.
Fortunately ARM and x86 share same vDSO name, and we can define macros
for variations.

user ABI vDSO name
?????????????????????????????
aarch64 linux-vdso.so.1
arm linux-vdso.so.1
ia64 linux-gate.so.1
mips linux-vdso.so.1
ppc/32 linux-vdso32.so.1
ppc/64 linux-vdso64.so.1
s390 linux-vdso32.so.1
s390x linux-vdso64.so.1
sh linux-gate.so.1
i386 linux-gate.so.1
x86-64 linux-vdso.so.1
x86/x32 linux-vdso.so.1

While unfortunately, looks like ARM vDSO doesn't have getcpu(). In that
case, we might roll back to syscall(__NR_getcpu)?

aarch64 functions
The table below lists the symbols exported by the vDSO.

symbol version
--------------------------------------
__kernel_rt_sigreturn LINUX_2.6.39
__kernel_gettimeofday LINUX_2.6.39
__kernel_clock_gettime LINUX_2.6.39
__kernel_clock_getres LINUX_2.6.39

https://man7.org/linux/man-pages/man7/vdso.7.html

> > static int next_cpu(int cpu)
> > @@ -205,6 +219,8 @@ int main(int argc, char *argv[])
> > struct kvm_vcpu *vcpu;
> > u32 cpu, rseq_cpu;
> >
> > + init_vdso();
> > +
> > /* Tell stdout not to buffer its content */
> > setbuf(stdout, NULL);
> >
> > @@ -253,7 +269,7 @@ int main(int argc, char *argv[])
> > * across the seq_cnt reads.
> > */
> > smp_rmb();
> > - sys_getcpu(&cpu);
> > + vdso_getcpu(&cpu, NULL, NULL);
> > rseq_cpu = rseq_current_cpu_raw();
> > smp_rmb();
> > } while (snapshot != atomic_read(&seq_cnt));
> >
>
> Thanks,
> Gavin
>