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

[Alexandru Elisei <alexandru.elisei@arm.com>]

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
> 

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