Re: [PATCH v2 1/2] KVM: arm64: Allow userspace to change ID_AA64PFR1_EL1

[Shaoqin Huang]

On 6/26/24 15:33, Shaoqin Huang wrote:
> Hi Marc,
>
> On 6/21/24 15:53, Marc Zyngier wrote:
> > On Fri, 21 Jun 2024 07:17:57 +0100,
> > Shaoqin Huang <shahuang@xxxxxxxxxx> wrote:
> > > Hi Marc,
> > >
> > > On 6/18/24 15:39, Marc Zyngier wrote:
> > > > On Tue, 18 Jun 2024 07:38:06 +0100,
> > > > Shaoqin Huang <shahuang@xxxxxxxxxx> wrote:
> > > > > Allow userspace to change the guest-visible value of the register with
> > > > > some severe limitation:
> > > > >
> > > > >     - No changes to features not virtualized by KVM (MPAM_frac, 
> > > > > RAS_frac)
> > > > > ---
> > > > >    arch/arm64/kvm/sys_regs.c | 3 ++-
> > > > >    1 file changed, 2 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > > > > index 22b45a15d068..bead81867bce 100644
> > > > > --- a/arch/arm64/kvm/sys_regs.c
> > > > > +++ b/arch/arm64/kvm/sys_regs.c
> > > > > @@ -2306,7 +2306,8 @@ static const struct sys_reg_desc 
> > > > > sys_reg_descs[] = {
> > > > >               ID_AA64PFR0_EL1_GIC |
> > > > >               ID_AA64PFR0_EL1_AdvSIMD |
> > > > >               ID_AA64PFR0_EL1_FP), },
> > > > > -    ID_SANITISED(ID_AA64PFR1_EL1),
> > > > > +    ID_WRITABLE(ID_AA64PFR1_EL1, ~(ID_AA64PFR1_EL1_RAS_frac |
> > > > > +                       ID_AA64PFR1_EL1_MPAM_frac)),
> > > > >        ID_UNALLOCATED(4,2),
> > > > >        ID_UNALLOCATED(4,3),
> > > > >        ID_WRITABLE(ID_AA64ZFR0_EL1, ~ID_AA64ZFR0_EL1_RES0),
> > > >
> > > > This isn't a valid patch.
> > > >
> > > > Furthermore, how about all the other features that may or may not be
> > > > currently handled by KVM? Please see [1] and make sure that all
> > > > existing fields have a known behaviour (a combination of masked,
> > > > preserved, capped, writable or read-only).
> > > >
> > > > I can at least see problems with MTE_frac and MTEX, plus all the other
> > > > things that KVM doesn't know how to save/restore (THE, GCS, NMI...).
> > > >
> > > > What I asked you to handle the whole register, I really meant it.
> > >
> > > I currently only found the BT and SSBS fields can be written without
> > > any unknown behavior.
> >
> > I can only assume you haven't looked hard enough.
> >
> > > All other fields in the ID_AA64PFR1_EL1 are either not supported by
> > > KVM or the field involved with other register and KVM don't know how
> > > to handle them.
> >
> > Why can't CSV2_frac be writable? Why can't most of the other fields be
> > hidden depending on the VM configuration, as pointed out above?
>
> I looked at the "struct arm64_ftr_bits ftr_id_aa64pfr1[]" in the 
> kernel/cpufeature.c, I don't see the CSV2_frac has been supported on ARM 
> bare-mental. In this situation, can we first support it in KVM? If so, 
> how can we do that, I don't understand that, could you give me some 
> hints about that.

More description about the CSV2_frac.

static const struct arm64_ftr_bits ftr_id_aa64pfr1[] = {
	ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_SME),
		       FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_EL1_SME_SHIFT, 4, 0),
	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, 
ID_AA64PFR1_EL1_MPAM_frac_SHIFT, 4, 0),
	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, 
ID_AA64PFR1_EL1_RAS_frac_SHIFT, 4, 0),
	ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_MTE),
		       FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_EL1_MTE_SHIFT, 4, 
ID_AA64PFR1_EL1_MTE_NI),
	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, 
ID_AA64PFR1_EL1_SSBS_SHIFT, 4, ID_AA64PFR1_EL1_SSBS_NI),
	ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_BTI),
				    FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_EL1_BT_SHIFT, 4, 0),
	ARM64_FTR_END,
};

If we make the CSV2_frac writable, there isn't definition about 
CSV2_frac in the ftr_id_aa64pfr1. And the KVM really depends on the 
ftr_id_aa64pfr1 in the arm64_check_features() function. We're missing 
the CSV2_frac.

So the arm64_check_features() will not check the CSV2_frac if it's 
writable, and the user can write any value into it.

Thanks,
Shaoqin

> Other fields are same with CSV2_frac I think. The KVM don't know the 
> configuration about them. Why we should allow them writable and hidden 
> them right now? Instead of just make them still unwrittable? >
> Thanks,
> Shaoqin
>
> >     M.

--
Shaoqin