diff mbox series

[2/2] KVM: arm64: Use get_raz_reg() for userspace reads of PMSWINC_EL0

Message ID 20210927124911.191729-3-alexandru.elisei@arm.com (mailing list archive)
State New, archived
Headers show
Series KVM: arm64: Minor improvements to RAZ register handling | expand

Commit Message

Alexandru Elisei Sept. 27, 2021, 12:49 p.m. UTC
PMSWINC_EL0 is a write-only register and was initially part of the VCPU
register state, but was later removed in commit 7a3ba3095a32 ("KVM:
arm64: Remove PMSWINC_EL0 shadow register"). To prevent regressions, the
register was kept accessible from userspace as Read-As-Zero (RAZ).

The read function that is used to handle userspace reads of this
register is get_raz_id_reg(), which, while technically correct, as it
returns 0, it is not semantically correct, as PMSWINC_EL0 is not an ID
register as the function name suggests.

Add a new function, get_raz_reg(), to use it as the accessor for
PMSWINC_EL0, as to not conflate get_raz_id_reg() to handle other types
of registers.

No functional change intended.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
---
 arch/arm64/kvm/sys_regs.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

Comments

Andrew Jones Sept. 30, 2021, 1:29 p.m. UTC | #1
On Mon, Sep 27, 2021 at 01:49:11PM +0100, Alexandru Elisei wrote:
> PMSWINC_EL0 is a write-only register and was initially part of the VCPU
> register state, but was later removed in commit 7a3ba3095a32 ("KVM:
> arm64: Remove PMSWINC_EL0 shadow register"). To prevent regressions, the
> register was kept accessible from userspace as Read-As-Zero (RAZ).
> 
> The read function that is used to handle userspace reads of this
> register is get_raz_id_reg(), which, while technically correct, as it
> returns 0, it is not semantically correct, as PMSWINC_EL0 is not an ID
> register as the function name suggests.
> 
> Add a new function, get_raz_reg(), to use it as the accessor for
> PMSWINC_EL0, as to not conflate get_raz_id_reg() to handle other types
> of registers.
> 
> No functional change intended.
> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> ---
>  arch/arm64/kvm/sys_regs.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 4adda8bf3168..1be827740f87 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1285,6 +1285,15 @@ static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
>  	return __set_id_reg(vcpu, rd, uaddr, true);
>  }
>  
> +static int get_raz_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> +		       const struct kvm_one_reg *reg, void __user *uaddr)
> +{
> +	const u64 id = sys_reg_to_index(rd);
> +	const u64 val = 0;
> +
> +	return reg_to_user(uaddr, &val, id);
> +}
> +
>  static int set_wi_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
>  		      const struct kvm_one_reg *reg, void __user *uaddr)
>  {
> @@ -1647,7 +1656,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	 * previously (and pointlessly) advertised in the past...
>  	 */
>  	{ PMU_SYS_REG(SYS_PMSWINC_EL0),
> -	  .get_user = get_raz_id_reg, .set_user = set_wi_reg,
> +	  .get_user = get_raz_reg, .set_user = set_wi_reg,
>  	  .access = access_pmswinc, .reset = NULL },
>  	{ PMU_SYS_REG(SYS_PMSELR_EL0),
>  	  .access = access_pmselr, .reset = reset_pmselr, .reg = PMSELR_EL0 },
> -- 
> 2.33.0
>

What about replacing get_raz_id_reg() with this new function? Do really need
both?

Thanks,
drew
Alexandru Elisei Oct. 6, 2021, 2:49 p.m. UTC | #2
Hi Drew,

Thank you for the review!

On Thu, Sep 30, 2021 at 03:29:15PM +0200, Andrew Jones wrote:
> On Mon, Sep 27, 2021 at 01:49:11PM +0100, Alexandru Elisei wrote:
> > PMSWINC_EL0 is a write-only register and was initially part of the VCPU
> > register state, but was later removed in commit 7a3ba3095a32 ("KVM:
> > arm64: Remove PMSWINC_EL0 shadow register"). To prevent regressions, the
> > register was kept accessible from userspace as Read-As-Zero (RAZ).
> > 
> > The read function that is used to handle userspace reads of this
> > register is get_raz_id_reg(), which, while technically correct, as it
> > returns 0, it is not semantically correct, as PMSWINC_EL0 is not an ID
> > register as the function name suggests.
> > 
> > Add a new function, get_raz_reg(), to use it as the accessor for
> > PMSWINC_EL0, as to not conflate get_raz_id_reg() to handle other types
> > of registers.
> > 
> > No functional change intended.
> > 
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > ---
> >  arch/arm64/kvm/sys_regs.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 4adda8bf3168..1be827740f87 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -1285,6 +1285,15 @@ static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> >  	return __set_id_reg(vcpu, rd, uaddr, true);
> >  }
> >  
> > +static int get_raz_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> > +		       const struct kvm_one_reg *reg, void __user *uaddr)
> > +{
> > +	const u64 id = sys_reg_to_index(rd);
> > +	const u64 val = 0;
> > +
> > +	return reg_to_user(uaddr, &val, id);
> > +}
> > +
> >  static int set_wi_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> >  		      const struct kvm_one_reg *reg, void __user *uaddr)
> >  {
> > @@ -1647,7 +1656,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> >  	 * previously (and pointlessly) advertised in the past...
> >  	 */
> >  	{ PMU_SYS_REG(SYS_PMSWINC_EL0),
> > -	  .get_user = get_raz_id_reg, .set_user = set_wi_reg,
> > +	  .get_user = get_raz_reg, .set_user = set_wi_reg,
> >  	  .access = access_pmswinc, .reset = NULL },
> >  	{ PMU_SYS_REG(SYS_PMSELR_EL0),
> >  	  .access = access_pmselr, .reset = reset_pmselr, .reg = PMSELR_EL0 },
> > -- 
> > 2.33.0
> >
> 
> What about replacing get_raz_id_reg() with this new function? Do really need
> both?

I thought about that when writing this patch. I ultimately decided against it
because changing the get_user accessor to be get_raz_reg() instead of
get_raz_id_reg() would break the symmetry with set_user, which needs to stay
set_raz_id_reg(), and cannot be substituted with set_wi_reg() because that would
be a change in behaviour (set_raz_id_reg() checks that val == 0, set_wi_reg()
doesn't).

I do agree that get_raz_id_reg() does the exact same thing as get_raz_reg(), but
in a more roundabout manner. So if you still feel that I should use
get_raz_reg() instead, I'll do that for the next iteration of the series. What
do you think?

Thanks,
Alex

> 
> Thanks,
> drew
>
Andrew Jones Oct. 6, 2021, 3:23 p.m. UTC | #3
On Wed, Oct 06, 2021 at 03:49:19PM +0100, Alexandru Elisei wrote:
> Hi Drew,
> 
> Thank you for the review!
> 
> On Thu, Sep 30, 2021 at 03:29:15PM +0200, Andrew Jones wrote:
> > On Mon, Sep 27, 2021 at 01:49:11PM +0100, Alexandru Elisei wrote:
> > > PMSWINC_EL0 is a write-only register and was initially part of the VCPU
> > > register state, but was later removed in commit 7a3ba3095a32 ("KVM:
> > > arm64: Remove PMSWINC_EL0 shadow register"). To prevent regressions, the
> > > register was kept accessible from userspace as Read-As-Zero (RAZ).
> > > 
> > > The read function that is used to handle userspace reads of this
> > > register is get_raz_id_reg(), which, while technically correct, as it
> > > returns 0, it is not semantically correct, as PMSWINC_EL0 is not an ID
> > > register as the function name suggests.
> > > 
> > > Add a new function, get_raz_reg(), to use it as the accessor for
> > > PMSWINC_EL0, as to not conflate get_raz_id_reg() to handle other types
> > > of registers.
> > > 
> > > No functional change intended.
> > > 
> > > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > > ---
> > >  arch/arm64/kvm/sys_regs.c | 11 ++++++++++-
> > >  1 file changed, 10 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > > index 4adda8bf3168..1be827740f87 100644
> > > --- a/arch/arm64/kvm/sys_regs.c
> > > +++ b/arch/arm64/kvm/sys_regs.c
> > > @@ -1285,6 +1285,15 @@ static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> > >  	return __set_id_reg(vcpu, rd, uaddr, true);
> > >  }
> > >  
> > > +static int get_raz_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> > > +		       const struct kvm_one_reg *reg, void __user *uaddr)
> > > +{
> > > +	const u64 id = sys_reg_to_index(rd);
> > > +	const u64 val = 0;
> > > +
> > > +	return reg_to_user(uaddr, &val, id);
> > > +}
> > > +
> > >  static int set_wi_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> > >  		      const struct kvm_one_reg *reg, void __user *uaddr)
> > >  {
> > > @@ -1647,7 +1656,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> > >  	 * previously (and pointlessly) advertised in the past...
> > >  	 */
> > >  	{ PMU_SYS_REG(SYS_PMSWINC_EL0),
> > > -	  .get_user = get_raz_id_reg, .set_user = set_wi_reg,
> > > +	  .get_user = get_raz_reg, .set_user = set_wi_reg,
> > >  	  .access = access_pmswinc, .reset = NULL },
> > >  	{ PMU_SYS_REG(SYS_PMSELR_EL0),
> > >  	  .access = access_pmselr, .reset = reset_pmselr, .reg = PMSELR_EL0 },
> > > -- 
> > > 2.33.0
> > >
> > 
> > What about replacing get_raz_id_reg() with this new function? Do really need
> > both?
> 
> I thought about that when writing this patch. I ultimately decided against it
> because changing the get_user accessor to be get_raz_reg() instead of
> get_raz_id_reg() would break the symmetry with set_user, which needs to stay
> set_raz_id_reg(), and cannot be substituted with set_wi_reg() because that would
> be a change in behaviour (set_raz_id_reg() checks that val == 0, set_wi_reg()
> doesn't).
> 
> I do agree that get_raz_id_reg() does the exact same thing as get_raz_reg(), but
> in a more roundabout manner. So if you still feel that I should use
> get_raz_reg() instead, I'll do that for the next iteration of the series. What
> do you think?

I'd prefer we avoid maintaining two implementations of the same
functionality. If we want to keep the symmetry with set_raz_id_reg,
then we could implement get_raz_id_reg as 'return get_raz_reg()'.

Thanks,
drew
Alexandru Elisei Oct. 6, 2021, 3:35 p.m. UTC | #4
Hi Drew,

On Wed, Oct 06, 2021 at 05:23:02PM +0200, Andrew Jones wrote:
> On Wed, Oct 06, 2021 at 03:49:19PM +0100, Alexandru Elisei wrote:
> > Hi Drew,
> > 
> > Thank you for the review!
> > 
> > On Thu, Sep 30, 2021 at 03:29:15PM +0200, Andrew Jones wrote:
> > > On Mon, Sep 27, 2021 at 01:49:11PM +0100, Alexandru Elisei wrote:
> > > > PMSWINC_EL0 is a write-only register and was initially part of the VCPU
> > > > register state, but was later removed in commit 7a3ba3095a32 ("KVM:
> > > > arm64: Remove PMSWINC_EL0 shadow register"). To prevent regressions, the
> > > > register was kept accessible from userspace as Read-As-Zero (RAZ).
> > > > 
> > > > The read function that is used to handle userspace reads of this
> > > > register is get_raz_id_reg(), which, while technically correct, as it
> > > > returns 0, it is not semantically correct, as PMSWINC_EL0 is not an ID
> > > > register as the function name suggests.
> > > > 
> > > > Add a new function, get_raz_reg(), to use it as the accessor for
> > > > PMSWINC_EL0, as to not conflate get_raz_id_reg() to handle other types
> > > > of registers.
> > > > 
> > > > No functional change intended.
> > > > 
> > > > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > > > ---
> > > >  arch/arm64/kvm/sys_regs.c | 11 ++++++++++-
> > > >  1 file changed, 10 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > > > index 4adda8bf3168..1be827740f87 100644
> > > > --- a/arch/arm64/kvm/sys_regs.c
> > > > +++ b/arch/arm64/kvm/sys_regs.c
> > > > @@ -1285,6 +1285,15 @@ static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> > > >  	return __set_id_reg(vcpu, rd, uaddr, true);
> > > >  }
> > > >  
> > > > +static int get_raz_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> > > > +		       const struct kvm_one_reg *reg, void __user *uaddr)
> > > > +{
> > > > +	const u64 id = sys_reg_to_index(rd);
> > > > +	const u64 val = 0;
> > > > +
> > > > +	return reg_to_user(uaddr, &val, id);
> > > > +}
> > > > +
> > > >  static int set_wi_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> > > >  		      const struct kvm_one_reg *reg, void __user *uaddr)
> > > >  {
> > > > @@ -1647,7 +1656,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> > > >  	 * previously (and pointlessly) advertised in the past...
> > > >  	 */
> > > >  	{ PMU_SYS_REG(SYS_PMSWINC_EL0),
> > > > -	  .get_user = get_raz_id_reg, .set_user = set_wi_reg,
> > > > +	  .get_user = get_raz_reg, .set_user = set_wi_reg,
> > > >  	  .access = access_pmswinc, .reset = NULL },
> > > >  	{ PMU_SYS_REG(SYS_PMSELR_EL0),
> > > >  	  .access = access_pmselr, .reset = reset_pmselr, .reg = PMSELR_EL0 },
> > > > -- 
> > > > 2.33.0
> > > >
> > > 
> > > What about replacing get_raz_id_reg() with this new function? Do really need
> > > both?
> > 
> > I thought about that when writing this patch. I ultimately decided against it
> > because changing the get_user accessor to be get_raz_reg() instead of
> > get_raz_id_reg() would break the symmetry with set_user, which needs to stay
> > set_raz_id_reg(), and cannot be substituted with set_wi_reg() because that would
> > be a change in behaviour (set_raz_id_reg() checks that val == 0, set_wi_reg()
> > doesn't).
> > 
> > I do agree that get_raz_id_reg() does the exact same thing as get_raz_reg(), but
> > in a more roundabout manner. So if you still feel that I should use
> > get_raz_reg() instead, I'll do that for the next iteration of the series. What
> > do you think?
> 
> I'd prefer we avoid maintaining two implementations of the same
> functionality. If we want to keep the symmetry with set_raz_id_reg,
> then we could implement get_raz_id_reg as 'return get_raz_reg()'.

Agreed, I'll replace get_raz_id_reg() with get_raz_reg().

Thanks,
Alex

> 
> Thanks,
> drew
>
diff mbox series

Patch

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 4adda8bf3168..1be827740f87 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1285,6 +1285,15 @@  static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 	return __set_id_reg(vcpu, rd, uaddr, true);
 }
 
+static int get_raz_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
+		       const struct kvm_one_reg *reg, void __user *uaddr)
+{
+	const u64 id = sys_reg_to_index(rd);
+	const u64 val = 0;
+
+	return reg_to_user(uaddr, &val, id);
+}
+
 static int set_wi_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 		      const struct kvm_one_reg *reg, void __user *uaddr)
 {
@@ -1647,7 +1656,7 @@  static const struct sys_reg_desc sys_reg_descs[] = {
 	 * previously (and pointlessly) advertised in the past...
 	 */
 	{ PMU_SYS_REG(SYS_PMSWINC_EL0),
-	  .get_user = get_raz_id_reg, .set_user = set_wi_reg,
+	  .get_user = get_raz_reg, .set_user = set_wi_reg,
 	  .access = access_pmswinc, .reset = NULL },
 	{ PMU_SYS_REG(SYS_PMSELR_EL0),
 	  .access = access_pmselr, .reset = reset_pmselr, .reg = PMSELR_EL0 },