Re: dangers of current NEED_CPU_H, CONFIG_SOFTMMU, CONFIG_USER_ONLY

[Claudio Fontana <cfontana@suse.de>]

On 12/17/20 9:15 PM, Peter Maydell wrote:
> 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...

or should I? Using

#ifdef NEED_CPU_H
#ifdef CONFIG_SOFTMMU

seems to do what I expect. Is it wrong?

Thanks,

Claudio

> 
>> 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
>