Mark Cave-Ayland <mark.cave-ayland@xxxxxxxxxxxx> writes:
> QEMU's kvm_handle_debug() function identifies software breakpoints by checking
> for a value of 0 in kvm_debug_exit_arch's status field. Since this field isn't
LGTM, but let me start a discussion:
In arch/powerpc/include/uapi/asm/kvm.h I see the following:
/*
* Defines for h/w breakpoint, watchpoint (read, write or both) and
* software breakpoint.
* These are used as "type" in KVM_SET_GUEST_DEBUG ioctl and "status"
* for KVM_DEBUG_EXIT.
*/
#define KVMPPC_DEBUG_NONE 0x0
#define KVMPPC_DEBUG_BREAKPOINT (1UL << 1)
#define KVMPPC_DEBUG_WATCH_WRITE (1UL << 2)
#define KVMPPC_DEBUG_WATCH_READ (1UL << 3)
this seems to be biased towards the BookE implementation which uses
KVMPPC_DEBUG_BREAKPOINT to indicate a "hardware breakpoint" (i.e. Instruction
Address Compare - ISA v2 Book III-E, section 10.4.1), and then
KVMPPC_DEBUG_NONE ends up implicitly meaning "software breakpoint" for
Book3s PR/HV.
> explicitly set to 0 when the software breakpoint instruction is detected, any
> previous non-zero value present causes a hang in QEMU as it tries to process
> the breakpoint instruction incorrectly as a hardware breakpoint.
What QEMU does (again biased towards BookE) is to check the 'status'
field and treat both h/w breakpoints and watchpoints as hardware
breakpoints (which is correct in a sense) and then proceed to look for
s/w breakpoints:
if (arch_info->status) {
return kvm_handle_hw_breakpoint(cs, arch_info);
}
if (kvm_find_sw_breakpoint(cs, arch_info->address)) {
return kvm_handle_sw_breakpoint(cs, arch_info->address);
}
So I wonder if it would not be beneficial for future developers if we
drew the line and made a clear distinction between:
software breakpoints - triggered by a KVMPPC_INST_SW_BREAKPOINT execution
hardware breakpoints - triggered by some register match
Maybe by turning the first two defines into:
#define KVMPPC_DEBUG_SW_BREAKPOINT 0x0
#define KVMPPC_DEBUG_HW_BREAKPOINT (1UL << 1)
> Ensure that the kvm_debug_exit_arch status field is set to 0 when the software
> breakpoint instruction is detected (similar to the existing logic in booke.c
> and e500_emulate.c) to restore software breakpoint functionality under Book3S
> PR.
>
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@xxxxxxxxxxxx>
> ---
Anyway, the proposed patch fixes the issue.
Reviewed-by: Fabiano Rosas <farosas@xxxxxxxxxxxxx>
> arch/powerpc/kvm/emulate.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
> index bb4d09c1ad56..6fca38ca791f 100644
> --- a/arch/powerpc/kvm/emulate.c
> +++ b/arch/powerpc/kvm/emulate.c
> @@ -271,6 +271,7 @@ int kvmppc_emulate_instruction(struct kvm_run *run, struct kvm_vcpu *vcpu)
> */
> if (inst == KVMPPC_INST_SW_BREAKPOINT) {
> run->exit_reason = KVM_EXIT_DEBUG;
> + run->debug.arch.status = 0;
> run->debug.arch.address = kvmppc_get_pc(vcpu);
> emulated = EMULATE_EXIT_USER;
> advance = 0;
