Hi Dave,
On 18/02/2019 19:52, Dave Martin wrote:
> The reset_unknown() system register helper initialises a guest
> register to a distinctive junk value on vcpu reset, to help expose
> and debug deficient register initialisation within the guest.
>
> Some registers such as the SVE control register ZCR_EL1 contain a
> mixture of UNKNOWN fields and RES0 bits. For these,
> reset_unknown() does not work at present, since it sets all bits to
> junk values instead of just the wanted bits.
>
> There is no need to craft another special helper just for that,
> since reset_unknown() almost does the appropriate thing anyway.
> This patch takes advantage of the unused val field in struct
> sys_reg_desc to specify a mask of bits that should be initialised
> to zero instead of junk.
>
> All existing users of reset_unknown() do not (and should not)
> define a value for val, so they will implicitly set it to zero,
> resulting in all bits being made UNKNOWN by this function: thus,
> this patch makes no functional change for currently defined
> registers.
>
> Future patches will make use of non-zero val.
>
> Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> ---
> arch/arm64/kvm/sys_regs.h | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
> index 3b1bc7f..174ffc0 100644
> --- a/arch/arm64/kvm/sys_regs.h
> +++ b/arch/arm64/kvm/sys_regs.h
> @@ -56,7 +56,12 @@ struct sys_reg_desc {
> /* Index into sys_reg[], or 0 if we don't need to save it. */
> int reg;
>
> - /* Value (usually reset value) */
> + /*
> + * Value (usually reset value)
> + * For reset_unknown, each bit set to 1 in val is treated as
> + * RES0 in the register: the corresponding register bit is
> + * reset to 0 instead of "unknown".
> + */
Seeing there are users of this field, I find this a bit fragile. Is
there a reason not to add a separate "u64 res0_mask;" ?
The sys_reg_desc structures are instantiated once as constants for the
whole system rather than per VM/VCPU. Would it be really bad to add a
64bit field there?
> u64 val;
>
> /* Custom get/set_user functions, fallback to generic if NULL */
> @@ -92,7 +97,9 @@ static inline void reset_unknown(struct kvm_vcpu *vcpu,
> {
> BUG_ON(!r->reg);
> BUG_ON(r->reg >= NR_SYS_REGS);
> - __vcpu_sys_reg(vcpu, r->reg) = 0x1de7ec7edbadc0deULL;
> +
> + /* If non-zero, r->val specifies which register bits are RES0: */
> + __vcpu_sys_reg(vcpu, r->reg) = 0x1de7ec7edbadc0deULL & ~r->val;
> }
>
> static inline void reset_val(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
>
Cheers,
--
Julien Thierry
