Re: [PATCH v3 3/6] KVM: arm64: Allow guest to set the OSLK bit

[Oliver Upton <oupton@google.com>]

On Mon, Nov 29, 2021 at 5:51 AM Marc Zyngier <maz@kernel.org> wrote:
>
> On Tue, 23 Nov 2021 21:01:06 +0000,
> Oliver Upton <oupton@google.com> wrote:
> >
> > Allow writes to OSLAR and forward the OSLK bit to OSLSR. Do nothing with
> > the value for now.
> >
> > Reviewed-by: Reiji Watanabe <reijiw@google.com>
> > Signed-off-by: Oliver Upton <oupton@google.com>
> > ---
> >  arch/arm64/include/asm/sysreg.h |  6 ++++++
> >  arch/arm64/kvm/sys_regs.c       | 33 ++++++++++++++++++++++++++-------
> >  2 files changed, 32 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> > index 16b3f1a1d468..9fad61a82047 100644
> > --- a/arch/arm64/include/asm/sysreg.h
> > +++ b/arch/arm64/include/asm/sysreg.h
> > @@ -129,7 +129,13 @@
> >  #define SYS_DBGWCRn_EL1(n)           sys_reg(2, 0, 0, n, 7)
> >  #define SYS_MDRAR_EL1                        sys_reg(2, 0, 1, 0, 0)
> >  #define SYS_OSLAR_EL1                        sys_reg(2, 0, 1, 0, 4)
> > +
> > +#define SYS_OSLAR_OSLK                       BIT(0)
> > +
> >  #define SYS_OSLSR_EL1                        sys_reg(2, 0, 1, 1, 4)
> > +
> > +#define SYS_OSLSR_OSLK                       BIT(1)
> > +
> >  #define SYS_OSDLR_EL1                        sys_reg(2, 0, 1, 3, 4)
> >  #define SYS_DBGPRCR_EL1                      sys_reg(2, 0, 1, 4, 4)
> >  #define SYS_DBGCLAIMSET_EL1          sys_reg(2, 0, 7, 8, 6)
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 7bf350b3d9cd..5dbdb45d6d44 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -44,6 +44,10 @@
> >   * 64bit interface.
> >   */
> >
> > +static int reg_from_user(u64 *val, const void __user *uaddr, u64 id);
> > +static int reg_to_user(void __user *uaddr, const u64 *val, u64 id);
> > +static u64 sys_reg_to_index(const struct sys_reg_desc *reg);
> > +
> >  static bool read_from_write_only(struct kvm_vcpu *vcpu,
> >                                struct sys_reg_params *params,
> >                                const struct sys_reg_desc *r)
> > @@ -287,6 +291,24 @@ static bool trap_loregion(struct kvm_vcpu *vcpu,
> >       return trap_raz_wi(vcpu, p, r);
> >  }
> >
> > +static bool trap_oslar_el1(struct kvm_vcpu *vcpu,
> > +                        struct sys_reg_params *p,
> > +                        const struct sys_reg_desc *r)
> > +{
> > +     u64 oslsr;
> > +
> > +     if (!p->is_write)
> > +             return read_from_write_only(vcpu, p, r);
> > +
> > +     /* Forward the OSLK bit to OSLSR */
> > +     oslsr = __vcpu_sys_reg(vcpu, OSLSR_EL1) & ~SYS_OSLSR_OSLK;
> > +     if (p->regval & SYS_OSLAR_OSLK)
> > +             oslsr |= SYS_OSLSR_OSLK;
> > +
> > +     __vcpu_sys_reg(vcpu, OSLSR_EL1) = oslsr;
> > +     return true;
> > +}
> > +
> >  static bool trap_oslsr_el1(struct kvm_vcpu *vcpu,
> >                          struct sys_reg_params *p,
> >                          const struct sys_reg_desc *r)
> > @@ -309,9 +331,10 @@ static int set_oslsr_el1(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> >       if (err)
> >               return err;
> >
> > -     if (val != rd->val)
> > +     if ((val & ~SYS_OSLSR_OSLK) != rd->val)
> >               return -EINVAL;
>
> This looks odd. It means that once I have set the lock from userspace,
> I can't clear it?

This does read weird, but I believe it makes sense still. rd->val is
the value of the register after warm reset, and does not store the
current value of the actual register. The true value is stashed in
kvm_cpu_context. Really, what I'm asserting here is that the only RW
bit is the OSLK bit. If any of the other bits are changed it should
return an error.

I can either add a comment or make a macro for the expected register
value (or both) to make this more clear.

--
Thanks,
Oliver

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