arm: Allow system registers for KVM guests to be changed by QEMU code

At the moment the Arm implementations of kvm_arch_{get,put}_registers()
don't support having QEMU change the values of system registers
(aka coprocessor registers for AArch32). This is because although
kvm_arch_get_registers() calls write_list_to_cpustate() to
update the CPU state struct fields (so QEMU code can read the
values in the usual way), kvm_arch_put_registers() does not
call write_cpustate_to_list(), meaning that any changes to
the CPU state struct fields will not be passed back to KVM.

The rationale for this design is documented in a comment in the
AArch32 kvm_arch_put_registers() -- writing the values in the
cpregs list into the CPU state struct is "lossy" because the
write of a register might not succeed, and so if we blindly
copy the CPU state values back again we will incorrectly
change register values for the guest. The assumption was that
no QEMU code would need to write to the registers.

However, when we implemented debug support for KVM guests, we
broke that assumption: the code to handle "set the guest up
to take a breakpoint exception" does so by updating various
guest registers including ESR_EL1.

Support this by making kvm_arch_put_registers() synchronize
CPU state back into the list. We sync only those registers
where the initial write succeeds, which should be sufficient.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Dongjiu Geng <gengdongjiu@huawei.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index f0334413ec..bfc05c796a 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -2535,18 +2535,25 @@ bool write_list_to_cpustate(ARMCPU *cpu);
 /**
  * write_cpustate_to_list:
  * @cpu: ARMCPU
+ * @kvm_sync: true if this is for syncing back to KVM
  *
  * For each register listed in the ARMCPU cpreg_indexes list, write
  * its value from the ARMCPUState structure into the cpreg_values list.
  * This is used to copy info from TCG's working data structures into
  * KVM or for outbound migration.
  *
+ * @kvm_sync is true if we are doing this in order to sync the
+ * register state back to KVM. In this case we will only update
+ * values in the list if the previous list->cpustate sync actually
+ * successfully wrote the CPU state. Otherwise we will keep the value
+ * that is in the list.
+ *
  * Returns: true if all register values were read correctly,
  * false if some register was unknown or could not be read.
  * Note that we do not stop early on failure -- we will attempt
  * reading all registers in the list.
  */
-bool write_cpustate_to_list(ARMCPU *cpu);
+bool write_cpustate_to_list(ARMCPU *cpu, bool kvm_sync);
 
 #define ARM_CPUID_TI915T      0x54029152
 #define ARM_CPUID_TI925T      0x54029252