Hi Fuad,
On Mon, Jan 24, 2022 at 8:29 AM Fuad Tabba <tabba@xxxxxxxxxx> wrote:
>
> Hi Reiji,
>
> On Thu, Jan 6, 2022 at 4:28 AM Reiji Watanabe <reijiw@xxxxxxxxxx> wrote:
> >
> > This patch lays the groundwork to make ID registers writable.
> >
> > Introduce struct id_reg_info for an ID register to manage the
> > register specific control of its value for the guest, and provide set
> > of functions commonly used for ID registers to make them writable.
> >
> > The id_reg_info is used to do register specific initialization,
> > validation of the ID register and etc. Not all ID registers must
> > have the id_reg_info. ID registers that don't have the id_reg_info
> > are handled in a common way that is applied to all ID registers.
> >
> > At present, changing an ID register from userspace is allowed only
> > if the ID register has the id_reg_info, but that will be changed
> > by the following patches.
> >
> > No ID register has the structure yet and the following patches
> > will add the id_reg_info for some ID registers.
> >
> > Signed-off-by: Reiji Watanabe <reijiw@xxxxxxxxxx>
> > ---
> > arch/arm64/kvm/sys_regs.c | 243 ++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 230 insertions(+), 13 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 80dc62f98ef0..971018288bee 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -265,6 +265,101 @@ static bool trap_raz_wi(struct kvm_vcpu *vcpu,
> > return read_zero(vcpu, p);
> > }
> >
> > +struct id_reg_info {
> > + u32 sys_reg; /* Register ID */
>
> Nit: Why not have the comment above, as for the other fields of this struct?
Yes, I will fix that.
(I placed the comment there just because the comment was short
enough to place it on the right side)
>
> > +
> > + /*
> > + * Limit value of the register for a vcpu. The value is the sanitized
> > + * system value with bits cleared for unsupported features for the
> > + * guest.
> > + */
>
> I think that rather than saying "with bits cleared for unsupported
> features", it might be better to rephrase along the lines of "with
> bits indicating" or "set/cleared to indicate", so that it applies to
> signed fields as well.
Thank you for the suggestion ! I will fix the comment.
>
> > + u64 vcpu_limit_val;
> > +
> > + /* Fields that we don't to validate by arm64_check_features. */
>
> Nit: Remove "to".
I will fix it.
>
> > + u64 ignore_mask;
> > +
> > + /* Initialization function of the id_reg_info */
> > + void (*init)(struct id_reg_info *id_reg);
> > + /*
> > + * This is an optional ID register specific validation function.
> > + * When userspace tries to set the ID register, arm64_check_features()
> > + * will check if the requested value indicates any features that cannot
> > + * be supported by KVM on the host. But, some ID register fields need
> > + * a special checking, and this function can be used for such fields.
> > + * e.g. When SVE is configured for a vCPU by KVM_ARM_VCPU_INIT,
> > + * ID_AA64PFR0_EL1.SVE shouldn't be set to 0 for the vCPU.
> > + * The validation function for ID_AA64PFR0_EL1 could be used to check
> > + * the field is consistent with SVE configuration.
> > + */
> > + int (*validate)(struct kvm_vcpu *vcpu, const struct id_reg_info *id_reg,
> > + u64 val);
> > +
> > + /*
> > + * Return a bitmask of the vCPU's ID register fields that are not
> > + * synced with saved (per VM) ID register value, which usually
> > + * indicates opt-in CPU features that is not configured for the vCPU.
>
> Nit: s/is/are
I will fix it.
>
>
> > + * ID registers are saved per VM, but some opt-in CPU features can
> > + * be configured per vCPU. The saved (per VM) values for such
> > + * features are for vCPUs with the features (and zero for
> > + * vCPUs without the features).
> > + * Return value of this function is used to handle such fields
> > + * for per vCPU ID register read/write request with saved per VM
> > + * ID register. See the __write_id_reg's comment for more detail.
> > + */
> > + u64 (*vcpu_mask)(const struct kvm_vcpu *vcpu,
> > + const struct id_reg_info *id_reg);
> > +};
> > +
> > +static void id_reg_info_init(struct id_reg_info *id_reg)
> > +{
> > + id_reg->vcpu_limit_val = read_sanitised_ftr_reg(id_reg->sys_reg);
> > + if (id_reg->init)
>
> If there is an id_reg then the init function is necessary, isn't it?
> Otherwise it doesn't seem to add more than the default handling. If
> that's right, should we instead ensure that init is always set?
The init function is optional (maybe I should comment that).
>
> > + id_reg->init(id_reg);
> > +}
> > +
> > +/*
> > + * An ID register that needs special handling to control the value for the
> > + * guest must have its own id_reg_info in id_reg_info_table.
> > + * (i.e. the reset value is different from the host's sanitized value,
> > + * the value is affected by opt-in features, some fields need specific
> > + * validation, etc.)
> > + */
> > +#define GET_ID_REG_INFO(id) (id_reg_info_table[IDREG_IDX(id)])
> > +static struct id_reg_info *id_reg_info_table[KVM_ARM_ID_REG_MAX_NUM] = {};
> > +
> > +static int validate_id_reg(struct kvm_vcpu *vcpu, u32 id, u64 val)
> > +{
> > + const struct id_reg_info *id_reg = GET_ID_REG_INFO(id);
> > + u64 limit, tmp_val;
> > + int err;
> > +
> > + if (id_reg) {
> > + limit = id_reg->vcpu_limit_val;
> > + /*
> > + * Replace the fields that are indicated in ignore_mask with
> > + * the value in the limit to not have arm64_check_features()
> > + * check the field in @val.
> > + */
> > + tmp_val = val & ~id_reg->ignore_mask;
> > + tmp_val |= (limit & id_reg->ignore_mask);
> > + } else {
> > + limit = read_sanitised_ftr_reg(id);
> > + tmp_val = val;
> > + }
> > +
> > + /* Check if the value indicates any feature that is not in the limit. */
> > + err = arm64_check_features(id, tmp_val, limit);
> > + if (err)
> > + return err;
> > +
> > + if (id_reg && id_reg->validate)
> > + /* Run the ID register specific validity check. */
> > + err = id_reg->validate(vcpu, id_reg, val);
> > +
> > + return err;
> > +}
> > +
> > /*
> > * ARMv8.1 mandates at least a trivial LORegion implementation, where all the
> > * RW registers are RES0 (which we can implement as RAZ/WI). On an ARMv8.0
> > @@ -1061,9 +1156,81 @@ static bool access_arch_timer(struct kvm_vcpu *vcpu,
> > return true;
> > }
> >
> > +static u64 read_kvm_id_reg(struct kvm *kvm, u32 id)
> > +{
> > + return kvm->arch.id_regs[IDREG_IDX(id)];
> > +}
> > +
> > +static int modify_kvm_id_reg(struct kvm *kvm, u32 id, u64 val,
> > + u64 preserve_mask)
> > +{
> > + u64 old, new;
> > +
> > + mutex_lock(&kvm->lock);
> > +
> > + old = kvm->arch.id_regs[IDREG_IDX(id)];
> > +
> > + /* Preserve the value at the bit position set in preserve_mask */
> > + new = old & preserve_mask;
> > + new |= (val & ~preserve_mask);
> > +
> > + /* Don't allow to modify ID register value after KVM_RUN on any vCPUs */
> > + if (kvm_vm_has_started(kvm) && new != old)
> > + return -EBUSY;
>
> This path doesn't release the lock. I see that it gets fixed in a
> future patch, but it might be good to fix it here...
Thank you for catching it ! Yes, I will fix this.
>
> > +
> > + WRITE_ONCE(kvm->arch.id_regs[IDREG_IDX(id)], new);
> > + mutex_unlock(&kvm->lock);
> > +
> > + return 0;
> > +}
> > +
> > +static int write_kvm_id_reg(struct kvm *kvm, u32 id, u64 val)
> > +{
> > + return modify_kvm_id_reg(kvm, id, val, 0);
> > +}
> > +
> > +
> > +/*
> > + * KVM basically forces all vCPUs of the guest to have a uniform value for
> > + * each ID register (, which means KVM_SET_ONE_REG for a vCPU affects all
> > + * the vCPUs of the guest), and the id_regs[] of kvm_arch holds values
> > + * of ID registers for the guest. However, there is an exception for
> > + * ID register fields corresponding to CPU features that can be
> > + * configured per vCPU by KVM_ARM_VCPU_INIT, or etc (e.g. PMUv3, SVE, etc).
> > + * For such fields, all vCPUs that have the feature will have a non-zero
> > + * uniform value (, which can be updated by userspace), but the vCPUs that
>
> Nit: uneven nesting of parentheses and commas :)
I will fix it.
> > + * don't have the feature will have zero for the fields.
> > + * Values that @id_regs holds are for vCPUs that have such features. So,
> > + * to get the ID register value for a vCPU that doesn't have those features,
> > + * the corresponding fields in id_regs[] needs to be cleared.
> > + * A bitmask of the fields are provided by id_reg_info's vcpu_mask(), and
> > + * __write_id_reg() and __read_id_reg() take care of those fields using
> > + * the bitmask.
> > + */
> > +static int __write_id_reg(struct kvm_vcpu *vcpu, u32 id, u64 val)
> > +{
> > + const struct id_reg_info *id_reg = GET_ID_REG_INFO(id);
> > + u64 mask = 0;
> > +
> > + if (id_reg && id_reg->vcpu_mask)
> > + mask = id_reg->vcpu_mask(vcpu, id_reg);
> > +
> > + /*
> > + * Update the ID register for the guest with @val, except for fields
> > + * that are set in the mask, which indicates fields for opt-in
> > + * features that are not configured for the vCPU.
> > + */
> > + return modify_kvm_id_reg(vcpu->kvm, id, val, mask);
> > +}
> > +
> > static u64 __read_id_reg(const struct kvm_vcpu *vcpu, u32 id)
> > {
> > - u64 val = vcpu->kvm->arch.id_regs[IDREG_IDX(id)];
> > + const struct id_reg_info *id_reg = GET_ID_REG_INFO(id);
> > + u64 val = read_kvm_id_reg(vcpu->kvm, id);
> > +
> > + if (id_reg && id_reg->vcpu_mask)
> > + /* Clear fields for opt-in features that are not configured. */
> > + val &= ~(id_reg->vcpu_mask(vcpu, id_reg));
> > switch (id) {
> > case SYS_ID_AA64PFR0_EL1:
> > @@ -1222,12 +1389,7 @@ static int set_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
> > return 0;
> > }
> >
> > -/*
> > - * cpufeature ID register user accessors
> > - *
> > - * For now, these registers are immutable for userspace, so for set_id_reg()
> > - * we don't allow the effective value to be changed.
> > - */
> > +/* cpufeature ID register user accessors */
> > static int __get_id_reg(const struct kvm_vcpu *vcpu,
> > const struct sys_reg_desc *rd, void __user *uaddr,
> > bool raz)
> > @@ -1238,11 +1400,31 @@ static int __get_id_reg(const struct kvm_vcpu *vcpu,
> > return reg_to_user(uaddr, &val, id);
> > }
> >
> > +/*
> > + * Check if the given id indicates AArch32 ID register encoding.
> > + */
> > +static bool is_aarch32_id_reg(u32 id)
> > +{
> > + u32 crm, op2;
> > +
> > + if (!is_id_reg(id))
> > + return false;
> > +
> > + crm = sys_reg_CRm(id);
> > + op2 = sys_reg_Op2(id);
> > + if (crm == 1 || crm == 2 || (crm == 3 && (op2 != 3 && op2 != 7)))
>
> Consistent with the Arm ARM "Table D12-2 System instruction encodings
> for non-Debug System register accesses"
>
> > + /* AArch32 ID register */
> > + return true;
> > +
> > + return false;
> > +}
> > +
> > static int __set_id_reg(struct kvm_vcpu *vcpu,
> > const struct sys_reg_desc *rd, void __user *uaddr,
> > bool raz)
> > {
> > const u64 id = sys_reg_to_index(rd);
> > + u32 encoding = reg_to_encoding(rd);
> > int err;
> > u64 val;
> >
> > @@ -1250,11 +1432,28 @@ static int __set_id_reg(struct kvm_vcpu *vcpu,
> > if (err)
> > return err;
> >
> > - /* This is what we mean by invariant: you can't change it. */
> > - if (val != read_id_reg(vcpu, rd, raz))
> > + if (val == read_id_reg(vcpu, rd, raz))
> > + /* The value is same as the current value. Nothing to do. */
> > + return 0;
> > +
> > + /*
> > + * Don't allow to modify the register's value if the register is raz,
> > + * or the reg doesn't have the id_reg_info.
> > + */
> > + if (raz || !GET_ID_REG_INFO(encoding))
> > return -EINVAL;
> >
> > - return 0;
> > + /*
> > + * Skip the validation of AArch32 ID registers if the system doesn't
> > + * 32bit EL0 (their value are UNKNOWN).
> > + */
> > + if (system_supports_32bit_el0() || !is_aarch32_id_reg(encoding)) {
> > + err = validate_id_reg(vcpu, encoding, val);
> > + if (err)
> > + return err;
> > + }
> > +
> > + return __write_id_reg(vcpu, encoding, val);
> > }
> >
> > static int get_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> > @@ -2816,6 +3015,20 @@ int kvm_arm_copy_sys_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
> > return write_demux_regids(uindices);
> > }
> >
> > +static void id_reg_info_init_all(void)
> > +{
> > + int i;
> > + struct id_reg_info *id_reg;
> > +
> > + for (i = 0; i < ARRAY_SIZE(id_reg_info_table); i++) {
> > + id_reg = (struct id_reg_info *)id_reg_info_table[i];
> > + if (!id_reg)
> > + continue;
> > +
> > + id_reg_info_init(id_reg);
> > + }
> > +}
> > +
> > void kvm_sys_reg_table_init(void)
> > {
> > unsigned int i;
> > @@ -2850,6 +3063,8 @@ void kvm_sys_reg_table_init(void)
> > break;
> > /* Clear all higher bits. */
> > cache_levels &= (1 << (i*3))-1;
> > +
> > + id_reg_info_init_all();
> > }
> >
> > /*
> > @@ -2862,11 +3077,12 @@ void set_default_id_regs(struct kvm *kvm)
> > u32 id;
> > const struct sys_reg_desc *rd;
> > u64 val;
> > + struct id_reg_info *idr;
> >
> > for (i = 0; i < ARRAY_SIZE(sys_reg_descs); i++) {
> > rd = &sys_reg_descs[i];
> > if (rd->access != access_id_reg)
> > - /* Not ID register, or hidden/reserved ID register */
> > + /* Not ID register or hidden/reserved ID register */
> > continue;
> >
> > id = reg_to_encoding(rd);
> > @@ -2874,7 +3090,8 @@ void set_default_id_regs(struct kvm *kvm)
> > /* Shouldn't happen */
> > continue;
> >
> > - val = read_sanitised_ftr_reg(id);
> > - kvm->arch.id_regs[IDREG_IDX(id)] = val;
> > + idr = GET_ID_REG_INFO(id);
> > + val = idr ? idr->vcpu_limit_val : read_sanitised_ftr_reg(id);
> > + (void)write_kvm_id_reg(kvm, id, val);
>
> Rather than ignoring the return value of write_kvm_id_reg(), wouldn't
> it be better if set_default_id_regs were to propagate it back to
> kvm_arch_init_vm in case there's a problem?
Since write_kvm_id_reg() should never return an error for this
case, returning an error to kvm_arch_init_vm() adds a practically
unnecessary error handling, which I would like to avoid.
So, how about putting WARN_ON_ONCE on its return value ?
Thanks,
Reiji
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