Re: [PATCH] xen: Don't cast away const-ness in vcpu_show_registers()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.02.2025 00:02, Andrew Cooper wrote:
> The final hunk is `(struct vcpu *)v` in disguise, expressed using a runtime
> pointer chase through memory and a technicality of the C type system to work
> around the fact that get_hvm_registers() strictly requires a mutable pointer.
> 
> For anyone interested, this is one reason why C cannot optimise any reads
> across sequence points, even for a function purporting to take a const object.
> 
> Anyway, have the function correctly state that it needs a mutable vcpu.  All
> callers have a mutable vCPU to hand, and it removes the runtime pointer chase
> in x86.
> 
> Make one style adjustment in ARM while adjusting the parameter type.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> CC: Michal Orzel <michal.orzel@xxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Julien Grall <julien@xxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
> CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
> 
> RISC-V and PPC don't have this helper yet, not even in stub form.
> 
> I expect there will be one objection to this patch.  Since the last time we
> fought over this, speculative vulnerabilities have demonstrated how dangerous
> pointer chases are, and this is a violation of Misra Rule 11.8, even if it's
> not reasonable for Eclair to be able to spot and reject it.

On these grounds
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
irrespective of the fact that a function of this name and purpose really, really
should be taking a pointer-to-const.

Considering ...

> --- a/xen/arch/arm/include/asm/domain.h
> +++ b/xen/arch/arm/include/asm/domain.h
> @@ -243,7 +243,7 @@ struct arch_vcpu
>  
>  }  __cacheline_aligned;
>  
> -void vcpu_show_registers(const struct vcpu *v);
> +void vcpu_show_registers(struct vcpu *v);

... this and ...

> --- a/xen/arch/x86/include/asm/domain.h
> +++ b/xen/arch/x86/include/asm/domain.h
> @@ -688,7 +688,7 @@ bool update_secondary_system_time(struct vcpu *v,
>  void force_update_secondary_system_time(struct vcpu *v,
>                                          struct vcpu_time_info *map);
>  
> -void vcpu_show_registers(const struct vcpu *v);
> +void vcpu_show_registers(struct vcpu *v);

... this are the same, and there's nothing different to expect for other
architectures, centralizing the declaration and then adding a comment to
cover the non-const property may be desirable. Else people like me might
forget that there was this change, and try to re-add the seemingly
missing const.

Jan