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@xxxxxxxxxx>
> ---
> 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 (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.
Thanks,
Alex
