linux-kernel - Re: [PATCH v1 1/5] KVM: arm/arm64: Remove kvm_mmio

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Date:   Wed, 12 Jun 2019 13:48:05 +0100
From:   James Morse <james.morse@....com>
To:     Zenghui Yu <yuzenghui@...wei.com>
Cc:     linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org, marc.zyngier@....com,
        catalin.marinas@....com, will.deacon@....com, acme@...nel.org,
        linuxarm@...wei.com, acme@...hat.com, peterz@...radead.org,
        alexander.shishkin@...ux.intel.com, mingo@...hat.com,
        ganapatrao.kulkarni@...ium.com, namhyung@...nel.org,
        jolsa@...hat.com, xiexiangyou@...wei.com
Subject: Re: [PATCH v1 1/5] KVM: arm/arm64: Remove kvm_mmio_emulate tracepoint

Hi,

On 12/06/2019 10:08, Zenghui Yu wrote:
> In current KVM/ARM code, no one will invoke trace_kvm_mmio_emulate().
> Remove this TRACE_EVENT definition.

Oooer. We can't just go removing these things, they are visible to user-space.

I recall an article on this: https://lwn.net/Articles/737530/
"Another attempt to address the tracepoint ABI problem"

I agree this is orphaned, it was added by commit 45e96ea6b369 ("KVM: ARM: Handle I/O
aborts"), but there never was a caller.

The problem with removing it is /sys/kernel/debug/tracing/events/kvm/kvm_mmio_emulate
disappears. Any program relying on that being present (but useless) is now broken.


Thanks,

James


> diff --git a/virt/kvm/arm/trace.h b/virt/kvm/arm/trace.h
> index 204d210..8b7dff2 100644
> --- a/virt/kvm/arm/trace.h
> +++ b/virt/kvm/arm/trace.h
> @@ -114,27 +114,6 @@
>  		  __entry->type, __entry->vcpu_idx, __entry->irq_num, __entry->level)
>  );
>  
> -TRACE_EVENT(kvm_mmio_emulate,
> -	TP_PROTO(unsigned long vcpu_pc, unsigned long instr,
> -		 unsigned long cpsr),
> -	TP_ARGS(vcpu_pc, instr, cpsr),
> -
> -	TP_STRUCT__entry(
> -		__field(	unsigned long,	vcpu_pc		)
> -		__field(	unsigned long,	instr		)
> -		__field(	unsigned long,	cpsr		)
> -	),
> -
> -	TP_fast_assign(
> -		__entry->vcpu_pc		= vcpu_pc;
> -		__entry->instr			= instr;
> -		__entry->cpsr			= cpsr;
> -	),
> -
> -	TP_printk("Emulate MMIO at: 0x%08lx (instr: %08lx, cpsr: %08lx)",
> -		  __entry->vcpu_pc, __entry->instr, __entry->cpsr)
> -);
> -
>  TRACE_EVENT(kvm_unmap_hva_range,
>  	TP_PROTO(unsigned long start, unsigned long end),
>  	TP_ARGS(start, end),
>