Re: dangers of current NEED_CPU_H, CONFIG_SOFTMMU, CONFIG_USER_ONLY (was: [PATCH v11 7/7] cpu: introduce cpu_accel_instance_init)

[Peter Maydell]

On Thu, 17 Dec 2020 at 19:46, Claudio Fontana <cfontana@suse.de> wrote:
>
> Hi,
>
> I would like to highlight the current dangerous state of NEED_CPU_H / 
> CONFIG_SOFTMMU / CONFIG_USER_ONLY.

> So our struct TcgCpuOperations in include/hw/core/cpu.h,
> which contains after this series:
>
> #ifndef CONFIG_USER_ONLY
>     /**
>      * @do_transaction_failed: Callback for handling failed memory 
> transactions
>      * (ie bus faults or external aborts; not MMU faults)
>      */
>     void (*do_transaction_failed)(CPUState *cpu, hwaddr physaddr, vaddr addr,
>                                   unsigned size, MMUAccessType access_type,
>                                   int mmu_idx, MemTxAttrs attrs,
>                                   MemTxResult response, uintptr_t retaddr);
>     /**
>      * @do_unaligned_access: Callback for unaligned access handling
>      */
>     void (*do_unaligned_access)(CPUState *cpu, vaddr addr,
>                                 MMUAccessType access_type,
>                                 int mmu_idx, uintptr_t retaddr);
> #endif /* !CONFIG_USER_ONLY */

Yeah, don't try to ifdef out struct fields in common-compiled code...

> Note that include/hw/core/cpu.h already uses CONFIG_USER_ONLY in other parts 
> of the header file, and we might have hidden problems as a result we (or at 
> least I) don't know about,
> because code is being compiled in for linux-user which explicitly should not 
> be compiled there.

The other CONFIG_USER_ONLY checks in that file are only
ifdeffing out prototypes for functions that exist only in
the softmmu build, or providing do-nothing stubs for functions
that are softmmu only. I think they're safe.

> There are multiple workarounds / fixes possible for my short term problem,
> but would it not be a good idea to fix this problem at its root once and for 
> all?

What's your proposal for fixing things ?

Incidentally, this should not be a problem for CONFIG_SOFTMMU,
because that is listed in include/exec/poison.h so trying to
use it in a common (not compiled-per-target) file will give you
a compile error. (So in theory we could make CONFIG_USER_ONLY
a poisoned identifier but that will require some work to
adjust places where we currently use it in "safe" ways...)

thanks
-- PMM