Re: [RFC PATCH v3 03/29] KVM: arm64: Introduce struct id_reg_info

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Hi Reiji,

On 11/19/21 5:47 AM, Reiji Watanabe wrote:
> Hi Eric,
> 
> On Thu, Nov 18, 2021 at 12:36 PM Eric Auger <eauger@xxxxxxxxxx> wrote:
>>
>> Hi Reiji,
>>
>> On 11/17/21 7:43 AM, Reiji Watanabe 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/include/asm/sysreg.h |   1 +
>>>  arch/arm64/kvm/sys_regs.c       | 226 ++++++++++++++++++++++++++++++--
>>>  2 files changed, 218 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
>>> index 16b3f1a1d468..597609f26331 100644
>>> --- a/arch/arm64/include/asm/sysreg.h
>>> +++ b/arch/arm64/include/asm/sysreg.h
>>> @@ -1197,6 +1197,7 @@
>>>  #define ICH_VTR_TDS_MASK     (1 << ICH_VTR_TDS_SHIFT)
>>>
>>>  #define ARM64_FEATURE_FIELD_BITS     4
>>> +#define ARM64_FEATURE_FIELD_MASK     ((1ull << ARM64_FEATURE_FIELD_BITS) - 1)
>>>
>>>  /* Create a mask for the feature bits of the specified feature. */
>>>  #define ARM64_FEATURE_MASK(x)        (GENMASK_ULL(x##_SHIFT + ARM64_FEATURE_FIELD_BITS - 1, x##_SHIFT))
>>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>>> index 5608d3410660..1552cd5581b7 100644
>>> --- a/arch/arm64/kvm/sys_regs.c
>>> +++ b/arch/arm64/kvm/sys_regs.c
>>> @@ -265,6 +265,181 @@ static bool trap_raz_wi(struct kvm_vcpu *vcpu,
>>>               return read_zero(vcpu, p);
>>>  }
>>>
>>> +/*
>>> + * A value for FCT_LOWER_SAFE must be zero and changing that will affect
>>> + * ftr_check_types of id_reg_info.
>>> + */
>>> +enum feature_check_type {
>>> +     FCT_LOWER_SAFE = 0,
>>> +     FCT_HIGHER_SAFE,
>>> +     FCT_HIGHER_OR_ZERO_SAFE,
>>> +     FCT_EXACT,
>>> +     FCT_EXACT_OR_ZERO_SAFE,
>>> +     FCT_IGNORE,     /* Don't check (any value is fine) */
>> Maybe you can remove the _SAFE suffix (EXACT does not have it).
> 
> I am inclined to keep 'SAFE' (otherwise, I am likely to forget
> if lower is safe or not).
> 
>> s/EXACT/EQUAL ?
> 
> I will fix that FCT_EXACT to FCT_EQUAL_SAFE.
> 
>>> +};
>>> +
>>> +static int arm64_check_feature_one(enum feature_check_type type, int val,
>>> +                                int limit)
>>> +{
>>> +     bool is_safe = false;
>>> +
>>> +     if (val == limit)
>>> +             return 0;
>> even if the type is unexpected?
> 
> I will remove it.
then you need to modify the handling of FCT_EXACT*.
> 
>>> +
>>> +     switch (type) {
>>> +     case FCT_LOWER_SAFE:
>>> +             is_safe = (val <= limit);
>>> +             break;
>>> +     case FCT_HIGHER_OR_ZERO_SAFE:
>>> +             if (val == 0) {
>>> +                     is_safe = true;
>>> +                     break;
>>> +             }
>>> +             fallthrough;
>>> +     case FCT_HIGHER_SAFE:
>>> +             is_safe = (val >= limit);
>>> +             break;
>>> +     case FCT_EXACT:
>>> +             break;
>>> +     case FCT_EXACT_OR_ZERO_SAFE:
>>> +             is_safe = (val == 0);
>>> +             break;
>>> +     case FCT_IGNORE:
>>> +             is_safe = true;
>>> +             break;
>>> +     default:
>>> +             WARN_ONCE(1, "Unexpected feature_check_type (%d)\n", type);
>>> +             break;
>>> +     }
>>> +
>>> +     return is_safe ? 0 : -1;
>>> +}
>>> +
>>> +#define      FCT_TYPE_MASK           0x7
>>> +#define      FCT_TYPE_SHIFT          1
>>> +#define      FCT_SIGN_MASK           0x1
>>> +#define      FCT_SIGN_SHIFT          0
>>> +#define      FCT_TYPE(val)   ((val >> FCT_TYPE_SHIFT) & FCT_TYPE_MASK)
>>> +#define      FCT_SIGN(val)   ((val >> FCT_SIGN_SHIFT) & FCT_SIGN_MASK)
>>> +
>>> +#define      MAKE_FCT(shift, type, sign)                             \
>>> +     ((u64)((((type) & FCT_TYPE_MASK) << FCT_TYPE_SHIFT) |   \
>>> +            (((sign) & FCT_SIGN_MASK) << FCT_SIGN_SHIFT)) << (shift))
>>> +
>>> +/* For signed field */
>>> +#define      S_FCT(shift, type)      MAKE_FCT(shift, type, 1)
>>> +/* For unigned field */
>>> +#define      U_FCT(shift, type)      MAKE_FCT(shift, type, 0)
>>> +
>>> +/*
>>> + * @val and @lim are both a value of the ID register. The function checks
>>> + * if all features indicated in @val can be supported for guests on the host,
>>> + * which supports features indicated in @lim. @check_types indicates how> + * features in the ID register needs to be checked.
>>> + * See comments for id_reg_info's ftr_check_types field for more detail.
>> What about RES0 fields which may exist? add a comment to reassure about
>> the fact they are properly handled if there are?
> 
> Any fields including RES0 should be checked based on check_types.
> I will explicitly state that in the comment.
> 
>>> + */
>>> +static int arm64_check_features(u64 check_types, u64 val, u64 lim)
>>> +{
>>> +     int i;
>>> +
>>> +     for (i = 0; i < 64; i += ARM64_FEATURE_FIELD_BITS) {
>>> +             u8 ftr_check = (check_types >> i) & ARM64_FEATURE_FIELD_MASK;
>>> +             bool is_sign = FCT_SIGN(ftr_check);
>>> +             enum feature_check_type fctype = FCT_TYPE(ftr_check);
>>> +             int fval, flim, ret;
>>> +
>>> +             fval = cpuid_feature_extract_field(val, i, is_sign);
>>> +             flim = cpuid_feature_extract_field(lim, i, is_sign);
>>> +
>>> +             ret = arm64_check_feature_one(fctype, fval, flim);
>>> +             if (ret)
>>> +                     return -E2BIG;
>>> +     }
>>> +     return 0;
>>> +}
>>> +
>>> +struct id_reg_info {
>>> +     u32     sys_reg;        /* Register ID */
>> use struct kernel-doc comments instead?
>>> +
>>> +     /*
>>> +      * Limit value of the register for a vcpu. The value is the sanitized
>>> +      * system value with bits cleared for unsupported features for the
>>> +      * guest.
>>> +      */
>>> +     u64     vcpu_limit_val;
>>> +
>>> +     /*
>>> +      * The ftr_check_types is comprised of a set of 4 bits fields.
>> nit: s/bits field/bit field here and below
> 
> I will fix them.
> 
>>> +      * Each 4 bits field is for a feature indicated by the same bits
>>> +      * field of the ID register and indicates how the feature support
>>> +      * for guests needs to be checked.
>>> +      * The bit 0 indicates that the corresponding ID register field
>>> +      * is signed(1) or unsigned(0).
>>> +      * The bits [3:1] hold feature_check_type for the field.
>>> +      * If all zero, all features in the ID register are treated as unsigned
>>> +      * fields and checked based on Principles of the ID scheme for fields
>>> +      * in ID registers (FCT_LOWER_SAFE of feature_check_type).
>> values set by the guest are checked against host ID field values
>> according to FCT_LOWER_SAFE test? You do not actually explicitly explain
>> what the check is about although this may be obvious for you?
> 
> How about this ?
> 
>         /*
>          * The ftr_check_types is comprised of a set of 4 bit fields.
>          * Each 4 bit field is for a feature indicated by the same bit field
>          * of the ID register and indicates how the field needs to be checked
>          * (by arm64_check_feature_one) against the host's ID field when
>          * userspace tries to set the register.
>          * The bit 0 indicates that the corresponding ID register field is
>          * signed(1) or unsigned(0). The bits [3:1] hold feature_check_type
>          * for the field (FCT_LOWER_SAFE == 0, etc).
>          * e.g. for ID_AA64PFR0_EL1.SVE(bits [35:32]), bits[35:32] of
>          * ftr_check_types for the register should be 0. It means the SVE
>          * field is treated as an unsigned field, and userspace can set the
>          * field to a equal or lower value than the host's ID field value.
>          */
yep sounds clearer to me.
> 
>>> +      */
>>> +     u64     ftr_check_types;
>>> +
>>> +     /* Initialization function of the id_reg_info */
>>> +     void (*init)(struct id_reg_info *id_reg);
>>> +
>>> +     /* Register specific validation function */
>> validation callback? it does not register anything. We have check
>> customization means already in ftr_check_types so it is difficult to
>> guess at that point why this cb is needed, all the more so it applies
>> after the ftr_checks.
> 
> I am going to add the following comment. Does it look clear enough for you ?
> 
>         /*
>          * 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. KVM_CREATE_DEVICE must be used to configure GICv3 for a guest.
>          * ID_AA64PFR0_EL1.GIC shouldn't be set to 1 unless GICv3 is configured.
>          * The validation function for ID_AA64PFR0_EL1 could be used to check
>          * the field is consistent with GICv3 configuration.
>          */
> 
>>> +     int (*validate)(struct kvm_vcpu *vcpu, const struct id_reg_info *id_reg,
>>> +                     u64 val);
>>> +
>>> +     /* Return the reset value of the register for the vCPU */
>>> +     u64 (*get_reset_val)(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)
>>> +             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 needs specific
>> s/needs/need
> 
> I will fix it.
> 
> Thank you for your review !
> 
> Regards
> Reiji
> 

Thanks

Eric




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