[AArch64] When unavailable, fetch VG from ptrace.

I was doing some SVE tests on system QEMU and noticed quite a few failures
related to inferior function calls. Any attempt to do an inferior function
call would result in the following:

Unable to set VG register.: Success.

This happens because, after an inferior function call, GDB attempts to restore
the regcache state and updates the SVE register in order. Since the Z registers
show up before the VG register, VG is still INVALID by the time the first Z
register is being updated. So when executing the following code in
aarch64_sve_set_vq:

if (reg_buf->get_register_status (AARCH64_SVE_VG_REGNUM) != REG_VALID)
  return false;

By returning false, we signal something is wrong, then we get to this:

  /* First store vector length to the thread.  This is done first to ensure the
     ptrace buffers read from the kernel are the correct size.  */
  if (!aarch64_sve_set_vq (tid, regcache))
    perror_with_name (_("Unable to set VG register."));

Ideally we'd always have a valid VG before attempting to set the Z registers,
but in this case the ordering of registers doesn't make that possible.

I considered reordering the registers to put VG before the Z registers, like
the DWARF numbering, but that would break backwards compatibility with
existing implementations. Also, the Z register numbering is pinned to the V
registers, and adding VG before Z would create a gap for non-SVE targets,
since we wouldn't be able to undefine VG for non-SVE targets.

As a compromise, it seems we can safely fetch the VG register value from
ptrace. The value in the kernel is likely the updated value anyway.

This patch fixed all the failures i saw in the testsuite and caused no further
regressions.

gdb/ChangeLog:

2020-03-19  Luis Machado  <luis.machado@linaro.org>

	* nat/aarch64-sve-linux-ptrace.c (aarch64_sve_set_vq): If vg is not
	valid, fetch vg value from ptrace.

1 files changed, 18 insertions, 3 deletions

diff --git a/gdb/nat/aarch64-sve-linux-ptrace.c b/gdb/nat/aarch64-sve-linux-ptrace.c
index b8f0711..2ce90cc 100644
--- a/gdb/nat/aarch64-sve-linux-ptrace.c
+++ b/gdb/nat/aarch64-sve-linux-ptrace.c
@@ -92,11 +92,26 @@ aarch64_sve_set_vq (int tid, uint64_t vq)
 bool
 aarch64_sve_set_vq (int tid, struct reg_buffer_common *reg_buf)
 {
+  uint64_t reg_vg = 0;
+
+  /* The VG register may not be valid if we've not collected any value yet.
+     This can happen, for example,  if we're restoring the regcache after an
+     inferior function call, and the VG register comes after the Z
+     registers.  */
   if (reg_buf->get_register_status (AARCH64_SVE_VG_REGNUM) != REG_VALID)
-    return false;
+  {
+    /* If vg is not available yet, fetch it from ptrace.  The VG value from
+       ptrace is likely the correct one.  */
+    uint64_t vq = aarch64_sve_get_vq (tid);
 
-  uint64_t reg_vg = 0;
-  reg_buf->raw_collect (AARCH64_SVE_VG_REGNUM, &reg_vg);
+    /* If something went wrong, just bail out.  */
+    if (vq == 0)
+      return false;
+
+    reg_vg = sve_vg_from_vq (vq);
+  }
+  else
+    reg_buf->raw_collect (AARCH64_SVE_VG_REGNUM, &reg_vg);
 
   return aarch64_sve_set_vq (tid, sve_vq_from_vg (reg_vg));
 }