Re: [PATCH v2 4/4] KVM: arm64: Remove PMSWINC_EL0 shadow register

[Marc Zyngier <maz@kernel.org>]

On Tue, 20 Jul 2021 17:44:32 +0100,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> Hi Marc,
> 
> On 7/19/21 5:56 PM, Marc Zyngier wrote:
> > Hi Alex,
> >
> > On 2021-07-19 17:35, Alexandru Elisei wrote:
> >> Hi Marc,
> >>
> >> On 7/19/21 1:39 PM, Marc Zyngier wrote:
> >>> We keep an entry for the PMSWINC_EL0 register in the vcpu structure,
> >>> while *never* writing anything there outside of reset.
> >>>
> >>> Given that the register is defined as write-only, that we always
> >>> trap when this register is accessed, there is little point in saving
> >>> anything anyway.
> >>>
> >>> Get rid of the entry, and save a mighty 8 bytes per vcpu structure.
> >>>
> >>> We still need to keep it exposed to userspace in order to preserve
> >>> backward compatibility with previously saved VMs. Since userspace
> >>> cannot expect any effect of writing to PMSWINC_EL0, treat the
> >>> register as RAZ/WI for the purpose of userspace access.
> >>>
> >>> Signed-off-by: Marc Zyngier <maz@kernel.org>
> >>> ---
> >>>  arch/arm64/include/asm/kvm_host.h |  1 -
> >>>  arch/arm64/kvm/sys_regs.c         | 21 ++++++++++++++++++++-
> >>>  2 files changed, 20 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/arch/arm64/include/asm/kvm_host.h
> >>> b/arch/arm64/include/asm/kvm_host.h
> >>> index 41911585ae0c..afc169630884 100644
> >>> --- a/arch/arm64/include/asm/kvm_host.h
> >>> +++ b/arch/arm64/include/asm/kvm_host.h
> >>> @@ -185,7 +185,6 @@ enum vcpu_sysreg {
> >>>      PMCNTENSET_EL0,    /* Count Enable Set Register */
> >>>      PMINTENSET_EL1,    /* Interrupt Enable Set Register */
> >>>      PMOVSSET_EL0,    /* Overflow Flag Status Set Register */
> >>> -    PMSWINC_EL0,    /* Software Increment Register */
> >>>      PMUSERENR_EL0,    /* User Enable Register */
> >>>
> >>>      /* Pointer Authentication Registers in a strict increasing order. */
> >>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> >>> index f22139658e48..a1f5101f49a3 100644
> >>> --- a/arch/arm64/kvm/sys_regs.c
> >>> +++ b/arch/arm64/kvm/sys_regs.c
> >>> @@ -1286,6 +1286,20 @@ 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 set_wi_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> >>> +              const struct kvm_one_reg *reg, void __user *uaddr)
> >>> +{
> >>> +    int err;
> >>> +    u64 val;
> >>> +
> >>> +    /* Perform the access even if we are going to ignore the value */
> >>> +    err = reg_from_user(&val, uaddr, sys_reg_to_index(rd));
> >>
> >> I don't understand why the read still happens if the value is ignored.
> >> Just so KVM
> >> preserves the previous behaviour and tells userspace there was an error?
> >
> > If userspace has given us a duff pointer, it needs to know about it.
> 
> Makes sense, thanks.
> 
> >
> >>> +    if (err)
> >>> +        return err;
> >>> +
> >>> +    return 0;
> >>> +}
> >>> +
> >>>  static bool access_ctr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
> >>>                 const struct sys_reg_desc *r)
> >>>  {
> >>> @@ -1629,8 +1643,13 @@ static const struct sys_reg_desc sys_reg_descs[] = {
> >>>        .access = access_pmcnten, .reg = PMCNTENSET_EL0 },
> >>>      { PMU_SYS_REG(SYS_PMOVSCLR_EL0),
> >>>        .access = access_pmovs, .reg = PMOVSSET_EL0 },
> >>> +    /*
> >>> +     * PM_SWINC_EL0 is exposed to userspace as RAZ/WI, as it was
> >>> +     * previously (and pointlessly) advertised in the past...
> >>> +     */
> >>>      { PMU_SYS_REG(SYS_PMSWINC_EL0),
> >>> -      .access = access_pmswinc, .reg = PMSWINC_EL0 },
> >>> +      .get_user = get_raz_id_reg, .set_user = set_wi_reg,
> >>
> >> In my opinion, the call chain to return 0 looks pretty confusing to me, as the
> >> functions seemed made for ID register accesses, and the leaf function,
> >> read_id_reg(), tries to match this register with a list of ID
> >> registers. Since we
> >> have already added a new function just for PMSWINC_EL0, I was
> >> wondering if adding
> >> another function, something like get_raz_reg(), would make more sense.
> >
> > In that case, I'd rather just kill get_raz_id_reg() and replace it with
> > this get_raz_reg(). If we trat something as RAZ, who cares whether it is
> > an idreg or not?
> 
> I agree, the Arm ARM doesn't make the distinction between ID
> registers and other system registers in the definition of RAZ, I
> don't think KVM should either. And the way read_id_reg() is written
> allows returning a value different than 0 even if raz is true, which
> in my opinion could only happen because of a bug in KVM.
> 
> I can have a go at writing the patch(es) on top of this series, if
> you want. At the moment I'm rewriting the KVM SPE series, so it will
> be a few weeks until I get around to doing it though.

We can do that at any time, it is just a cleanup without any guest or
userspace visible effect. If a get a spare hour, I'll have a
look. Otherwise, this can wait a bit.

Thanks,

	M.

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

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