[PATCH v6 3/6] KVM: arm64: Enable writable for ID_AA64DFR0_EL1 and ID_DFR0_EL1

[Jing Zhang]

Hi Cornelia,

On Thu, Jul 20, 2023 at 1:52 AM Cornelia Huck <cohuck at redhat.com> wrote:
>
> On Tue, Jul 18 2023, Jing Zhang <jingzhangos at google.com> wrote:
>
> > All valid fields in ID_AA64DFR0_EL1 and ID_DFR0_EL1 are writable
> > from usrespace with this change.
>
> Typo: s/usrespace/userspace/
Thanks.
>
> >
> > Signed-off-by: Jing Zhang <jingzhangos at google.com>
> > ---
> >  arch/arm64/kvm/sys_regs.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 053d8057ff1e..f33aec83f1b4 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -2008,7 +2008,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> >         .set_user = set_id_dfr0_el1,
> >         .visibility = aa32_id_visibility,
> >         .reset = read_sanitised_id_dfr0_el1,
> > -       .val = ID_DFR0_EL1_PerfMon_MASK, },
> > +       .val = GENMASK(63, 0), },
> >       ID_HIDDEN(ID_AFR0_EL1),
> >       AA32_ID_SANITISED(ID_MMFR0_EL1),
> >       AA32_ID_SANITISED(ID_MMFR1_EL1),
> > @@ -2057,7 +2057,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> >         .get_user = get_id_reg,
> >         .set_user = set_id_aa64dfr0_el1,
> >         .reset = read_sanitised_id_aa64dfr0_el1,
> > -       .val = ID_AA64DFR0_EL1_PMUVer_MASK, },
> > +       .val = GENMASK(63, 0), },
> >       ID_SANITISED(ID_AA64DFR1_EL1),
> >       ID_UNALLOCATED(5,2),
> >       ID_UNALLOCATED(5,3),
>
> How does userspace find out whether a given id reg is actually writable,
> other than trying to write to it?
>
No mechanism was provided to userspace to discover if a given idreg or
any fields of a given idreg is writable. The write to a readonly idreg
can also succeed (write ignored) without any error if what's written
is exactly the same as what the idreg holds or if it is a write to
AArch32 idregs on an AArch64-only system.
Not sure if it is worth adding an API to return the writable mask for
idregs, since we want to enable the writable for all allocated
unhidden idregs eventually.

Thanks,
Jing