Re: [PULL 08/41] target/arm: Set CTR_EL0.{IDC,DIC} for the 'max' CPU

[Peter Maydell]

On Wed, 17 Jan 2024 at 21:24, Fabiano Rosas <farosas@suse.de> wrote:
> Peter Maydell <peter.maydell@linaro.org> writes:
> > diff --git a/target/arm/tcg/cpu64.c b/target/arm/tcg/cpu64.c
> > index fcda99e1583..40e7a45166f 100644
> > --- a/target/arm/tcg/cpu64.c
> > +++ b/target/arm/tcg/cpu64.c
> > @@ -1105,6 +1105,16 @@ void aarch64_max_tcg_initfn(Object *obj)
> >      u = FIELD_DP32(u, CLIDR_EL1, LOUU, 0);
> >      cpu->clidr = u;
> >
> > +    /*
> > +     * Set CTR_EL0.DIC and IDC to tell the guest it doesnt' need to
> > +     * do any cache maintenance for data-to-instruction or
> > +     * instruction-to-guest coherence. (Our cache ops are nops.)
> > +     */
> > +    t = cpu->ctr;
> > +    t = FIELD_DP64(t, CTR_EL0, IDC, 1);
> > +    t = FIELD_DP64(t, CTR_EL0, DIC, 1);
> > +    cpu->ctr = t;
> > +
> >      t = cpu->isar.id_aa64isar0;
> >      t = FIELD_DP64(t, ID_AA64ISAR0, AES, 2);      /* FEAT_PMULL */
> >      t = FIELD_DP64(t, ID_AA64ISAR0, SHA1, 1);     /* FEAT_SHA1 */
>
> Hi, we're introducing new regression tests to migration and this patch
> shows up in the bisect of an issue. I need some help figuring out
> whether this is an actual regression or something else.
>
> The migration is TCG QEMU 8.2.0 -> TCG QEMU master.
>
> On the destination side (contains this patch) we're hitting this
> condition:
>
> bool write_list_to_cpustate(ARMCPU *cpu)
> {
> ...
>         /*
>          * Write value and confirm it reads back as written
>          * (to catch read-only registers and partially read-only
>          * registers where the incoming migration value doesn't match)
>          */
>         write_raw_cp_reg(&cpu->env, ri, v);
>         if (read_raw_cp_reg(&cpu->env, ri) != v) {
> --->        ok = false;
>         }

This is (among other things) effectively checking that the
source and destination CPU agree about the values of constant
registers like the ID registers, of which this is one.

The "max" CPU is a moving target, so you shouldn't expect
to be able to migrate across QEMU versions using it:
it can have different features and thus different ID
register values between versions (as well as potentially
different actual-register-state if the added feature adds
new state).

thanks
-- PMM