linux-kernel - Re: [PATCH] KVM: arm64: mark expected switch fall-through

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Date:   Mon, 29 Jul 2019 10:46:16 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Matteo Croce <mcroce@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: mark expected switch fall-through

On 28/07/2019 23:53, Matteo Croce wrote:
> Mark switch cases where we are expecting to fall through,
> fixes the following warning:
> 
> In file included from ./arch/arm64/include/asm/kvm_emulate.h:19,
>                  from arch/arm64/kvm/regmap.c:13:
> arch/arm64/kvm/regmap.c: In function ‘vcpu_write_spsr32’:
> ./arch/arm64/include/asm/kvm_hyp.h:31:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
>    asm volatile(ALTERNATIVE(__msr_s(r##nvh, "%x0"), \
>    ^~~
> ./arch/arm64/include/asm/kvm_hyp.h:46:31: note: in expansion of macro ‘write_sysreg_elx’
>  #define write_sysreg_el1(v,r) write_sysreg_elx(v, r, _EL1, _EL12)
>                                ^~~~~~~~~~~~~~~~
> arch/arm64/kvm/regmap.c:180:3: note: in expansion of macro ‘write_sysreg_el1’
>    write_sysreg_el1(v, SYS_SPSR);
>    ^~~~~~~~~~~~~~~~
> arch/arm64/kvm/regmap.c:181:2: note: here
>   case KVM_SPSR_ABT:
>   ^~~~
> In file included from ./arch/arm64/include/asm/cputype.h:132,
>                  from ./arch/arm64/include/asm/cache.h:8,
>                  from ./include/linux/cache.h:6,
>                  from ./include/linux/printk.h:9,
>                  from ./include/linux/kernel.h:15,
>                  from ./include/asm-generic/bug.h:18,
>                  from ./arch/arm64/include/asm/bug.h:26,
>                  from ./include/linux/bug.h:5,
>                  from ./include/linux/mmdebug.h:5,
>                  from ./include/linux/mm.h:9,
>                  from arch/arm64/kvm/regmap.c:11:
> ./arch/arm64/include/asm/sysreg.h:837:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
>   asm volatile("msr " __stringify(r) ", %x0"  \
>   ^~~
> arch/arm64/kvm/regmap.c:182:3: note: in expansion of macro ‘write_sysreg’
>    write_sysreg(v, spsr_abt);
>    ^~~~~~~~~~~~
> arch/arm64/kvm/regmap.c:183:2: note: here
>   case KVM_SPSR_UND:
>   ^~~~
> In file included from ./arch/arm64/include/asm/cputype.h:132,
>                  from ./arch/arm64/include/asm/cache.h:8,
>                  from ./include/linux/cache.h:6,
>                  from ./include/linux/printk.h:9,
>                  from ./include/linux/kernel.h:15,
>                  from ./include/asm-generic/bug.h:18,
>                  from ./arch/arm64/include/asm/bug.h:26,
>                  from ./include/linux/bug.h:5,
>                  from ./include/linux/mmdebug.h:5,
>                  from ./include/linux/mm.h:9,
>                  from arch/arm64/kvm/regmap.c:11:
> ./arch/arm64/include/asm/sysreg.h:837:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
>   asm volatile("msr " __stringify(r) ", %x0"  \
>   ^~~
> arch/arm64/kvm/regmap.c:184:3: note: in expansion of macro ‘write_sysreg’
>    write_sysreg(v, spsr_und);
>    ^~~~~~~~~~~~
> arch/arm64/kvm/regmap.c:185:2: note: here
>   case KVM_SPSR_IRQ:
>   ^~~~
> In file included from ./arch/arm64/include/asm/cputype.h:132,
>                  from ./arch/arm64/include/asm/cache.h:8,
>                  from ./include/linux/cache.h:6,
>                  from ./include/linux/printk.h:9,
>                  from ./include/linux/kernel.h:15,
>                  from ./include/asm-generic/bug.h:18,
>                  from ./arch/arm64/include/asm/bug.h:26,
>                  from ./include/linux/bug.h:5,
>                  from ./include/linux/mmdebug.h:5,
>                  from ./include/linux/mm.h:9,
>                  from arch/arm64/kvm/regmap.c:11:
> ./arch/arm64/include/asm/sysreg.h:837:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
>   asm volatile("msr " __stringify(r) ", %x0"  \
>   ^~~
> arch/arm64/kvm/regmap.c:186:3: note: in expansion of macro ‘write_sysreg’
>    write_sysreg(v, spsr_irq);
>    ^~~~~~~~~~~~
> arch/arm64/kvm/regmap.c:187:2: note: here
>   case KVM_SPSR_FIQ:
>   ^~~~
> 
> Signed-off-by: Matteo Croce <mcroce@...hat.com>
> ---
>  arch/arm64/kvm/regmap.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/kvm/regmap.c b/arch/arm64/kvm/regmap.c
> index 0d60e4f0af66..b376b2fdbf51 100644
> --- a/arch/arm64/kvm/regmap.c
> +++ b/arch/arm64/kvm/regmap.c
> @@ -178,12 +178,16 @@ void vcpu_write_spsr32(struct kvm_vcpu *vcpu, unsigned long v)
>  	switch (spsr_idx) {
>  	case KVM_SPSR_SVC:
>  		write_sysreg_el1(v, SYS_SPSR);
> +		/* fallthrough */
>  	case KVM_SPSR_ABT:
>  		write_sysreg(v, spsr_abt);
> +		/* fallthrough */
>  	case KVM_SPSR_UND:
>  		write_sysreg(v, spsr_und);
> +		/* fallthrough */
>  	case KVM_SPSR_IRQ:
>  		write_sysreg(v, spsr_irq);
> +		/* fallthrough */
>  	case KVM_SPSR_FIQ:
>  		write_sysreg(v, spsr_fiq);
>  	}
> 

That's absolutely the *WRONG* fix. Please see [1] for the real thing.

Thanks,

	M.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm.git/commit/?id=3d584a3c85d6fe2cf878f220d4ad7145e7f89218
-- 
Jazz is not dead, it just smells funny...