Re: [PATCH v3 8/8] KVM: arm64: Expose ID_AA64PFR2_EL1 to userspace and guests

[Marc Zyngier <maz@kernel.org>]

On Tue, 13 Aug 2024 11:57:10 +0100,
Joey Gouly <joey.gouly@arm.com> wrote:
> 
> Hello!
> 
> On Tue, Aug 13, 2024 at 11:44:00AM +0100, Marc Zyngier wrote:
> > Everything is now in place for a guest to "enjoy" FP8 support.
> > Expose ID_AA64PFR2_EL1 to both userspace and guests, with the
> > explicit restriction of only being able to clear FPMR.
> > 
> > All other features (MTE* at the time of writing) are hidden
> > and not writable.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/kvm/sys_regs.c | 16 +++++++++++++++-
> >  1 file changed, 15 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 51627add0a72..da6d017f24a1 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -1722,6 +1722,15 @@ static u64 read_sanitised_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
> >  	return val;
> >  }
> >  
> > +static u64 read_sanitised_id_aa64pfr2_el1(struct kvm_vcpu *vcpu,
> > +					  const struct sys_reg_desc *rd)
> > +{
> > +	u64 val = read_sanitised_ftr_reg(SYS_ID_AA64PFR2_EL1);
> > +
> > +	/* We only expose FPMR */
> > +	return val & ID_AA64PFR2_EL1_FPMR;
> > +}
> 
> Wondering why you're adding this function instead of extending __kvm_read_sanitised_id_reg()?
>
> > +
> >  #define ID_REG_LIMIT_FIELD_ENUM(val, reg, field, limit)			       \
> >  ({									       \
> >  	u64 __f_val = FIELD_GET(reg##_##field##_MASK, val);		       \
> > @@ -2381,7 +2390,12 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> >  		   ID_AA64PFR0_EL1_AdvSIMD |
> >  		   ID_AA64PFR0_EL1_FP), },
> >  	ID_SANITISED(ID_AA64PFR1_EL1),
> > -	ID_UNALLOCATED(4,2),
> > +	{ SYS_DESC(SYS_ID_AA64PFR2_EL1),
> > +	  .access	= access_id_reg,
> > +	  .get_user	= get_id_reg,
> > +	  .set_user	= set_id_reg,
> > +	  .reset	= read_sanitised_id_aa64pfr2_el1,
> > +	  .val		= ID_AA64PFR2_EL1_FPMR, },
> 
> Then I think this would just be ID_WRITABLE(ID_AA64PFR2_EL1, ID_AA64PFR2_EL1_FPMR).

Yeah, that's an interesting point. I'm afraid I have lost track of the
many helpers that have been added over time.

Something like this?

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index da6d017f24a1..2d1e45178422 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1539,6 +1539,10 @@ static u64 __kvm_read_sanitised_id_reg(const struct kvm_vcpu *vcpu,
 
 		val &= ~ARM64_FEATURE_MASK(ID_AA64PFR1_EL1_SME);
 		break;
+	case SYS_ID_AA64PFR2_EL1:
+		/* We only expose FPMR */
+		val &= ID_AA64PFR2_EL1_FPMR;
+		break;
 	case SYS_ID_AA64ISAR1_EL1:
 		if (!vcpu_has_ptrauth(vcpu))
 			val &= ~(ARM64_FEATURE_MASK(ID_AA64ISAR1_EL1_APA) |
@@ -1722,15 +1726,6 @@ static u64 read_sanitised_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
 	return val;
 }
 
-static u64 read_sanitised_id_aa64pfr2_el1(struct kvm_vcpu *vcpu,
-					  const struct sys_reg_desc *rd)
-{
-	u64 val = read_sanitised_ftr_reg(SYS_ID_AA64PFR2_EL1);
-
-	/* We only expose FPMR */
-	return val & ID_AA64PFR2_EL1_FPMR;
-}
-
 #define ID_REG_LIMIT_FIELD_ENUM(val, reg, field, limit)			       \
 ({									       \
 	u64 __f_val = FIELD_GET(reg##_##field##_MASK, val);		       \
@@ -2390,12 +2385,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 		   ID_AA64PFR0_EL1_AdvSIMD |
 		   ID_AA64PFR0_EL1_FP), },
 	ID_SANITISED(ID_AA64PFR1_EL1),
-	{ SYS_DESC(SYS_ID_AA64PFR2_EL1),
-	  .access	= access_id_reg,
-	  .get_user	= get_id_reg,
-	  .set_user	= set_id_reg,
-	  .reset	= read_sanitised_id_aa64pfr2_el1,
-	  .val		= ID_AA64PFR2_EL1_FPMR, },
+	ID_WRITABLE(ID_AA64PFR2_EL1, ID_AA64PFR2_EL1_FPMR),
 	ID_UNALLOCATED(4,3),
 	ID_WRITABLE(ID_AA64ZFR0_EL1, ~ID_AA64ZFR0_EL1_RES0),
 	ID_HIDDEN(ID_AA64SMFR0_EL1),

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.