On Fri, Sep 28, 2018 at 02:39:14PM +0100, 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 ununused 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 <[email protected]>
> ---
> arch/arm64/kvm/sys_regs.h | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
> index cd710f8..24bac06 100644
> --- a/arch/arm64/kvm/sys_regs.h
> +++ b/arch/arm64/kvm/sys_regs.h
> @@ -89,7 +89,9 @@ 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;
nit: it would be nice to document this feature on the val field in the
sys_reg_desc structure above as well.
> }
>
> static inline void reset_val(struct kvm_vcpu *vcpu, const struct
> sys_reg_desc *r)
> --
> 2.1.4
>
Thanks,
Christoffer
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