Re: [PATCH] KVM: PPC: Book3S PR: fix software breakpoints

[Fabiano Rosas <farosas@linux.ibm.com>]

Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk> 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@ilande.co.uk>
> ---

Anyway, the proposed patch fixes the issue.

Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>

>  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;