On 7/31/25 19:09, Nicola Vetrini wrote: > On 2025-07-31 18:05, Andrew Cooper wrote: >> On 31/07/2025 4:58 pm, Jan Beulich wrote: >>> On 31.07.2025 17:37, Andrew Cooper wrote: >>>> On 31/07/2025 4:16 pm, Dmytro Prokopchuk1 wrote: >>>>> MISRA Rule 13.1: Initializer lists shall not contain persistent side >>>>> effects. >>>>> >>>>> The violations occur because both the `GVA_INFO` and `TRACE_TIME` >>>>> macro >>>>> expansions include expressions with persistent side effects introduced >>>>> via inline assembly. >>>>> >>>>> In the case of `GVA_INFO`, the issue stems from the initializer list >>>>> containing a direct call to `current`, which evaluates to >>>>> `this_cpu(curr_vcpu)` and involves persistent side effects via the >>>>> `asm` statement. To resolve this, the side-effect-producing expression >>>>> is computed in a separate statement prior to the macro initialization: >>>>> >>>>> struct vcpu *current_vcpu = current; >>>>> >>>>> The computed value is passed into the `GVA_INFO(current_vcpu)` macro, >>>>> ensuring that the initializer is clean and free of such side effects. >>>>> >>>>> Similarly, the `TRACE_TIME` macro violates this rule when accessing >>>>> expressions like `current->vcpu_id` and `current->domain->domain_id`, >>>>> which also depend on `current` and inline assembly. To fix this, the >>>>> value of `current` is assigned to a temporary variable: >>>>> >>>>> struct vcpu *v = current; >>>>> >>>>> This temporary variable is then used to access `domain_id` and >>>>> `vcpu_id`. >>>>> This ensures that the arguments passed to the `TRACE_TIME` macro are >>>>> simple expressions free of persistent side effects. >>>>> >>>>> Signed-off-by: Dmytro Prokopchuk <dmytro_prokopch...@epam.com> >>>> The macro `current` specifically does not (and must not) have side >>>> effects. It is expected to behave like a plain `struct vcpu *current;` >>>> variable, and what Eclair is noticing is the thread-local machinery >>>> under this_cpu() (or in x86's case, get_current()). >>>> >>>> In ARM's case, it's literally reading the hardware thread pointer >>>> register. Can anything be done to tell Eclair that `this_cpu()` >>>> specifically does not have side effects? >>>> >>>> The only reason that GVA_INFO() and TRACE_TIME() are picked out is >>>> because they both contain embedded structure initialisation, and >>>> this is >>>> is actually an example where trying to comply with MISRA interferes >>>> with >>>> what is otherwise a standard pattern in Xen. >>> Irrespective of what you say, some of the changes here were eliminating >>> multiple adjacent uses of current, which - iirc - often the compiler >>> can't fold via CSE. >> >> Where we have mixed usage, sure. (I'm sure I've got a branch somewhere >> trying to add some more pure/const around to try and help out here, but >> I can't find it, and don't recall it being a major improvement either.) >> >> The real problem here is that there are a *very few* number of contexts >> where Eclair refuses to tolerate the use of `current` citing side >> effects, despite there being no side effects. >> >> That is the thing that breaks the principle of least surprise, and we >> ought to fix it by making Eclair happy with `current` everywhere, rather >> than force people to learn that 2 macros can't have a `current` in their >> parameter list. >> > > I'll take a look. Likely yes, by adding a handful of properties. There > are subtleties, though. >
Hi, Nicola. Did you have a chance to try configure Eclair to ignore this macro `this_cpu()`? Thanks. Dmytro
