Re: [PATCH 3/8] KVM: arm64: Map AArch32 cp15 register to AArch64 sysregs

[James Morse <james.morse@arm.com>]

Hi Marc,

On 02/11/2020 19:16, Marc Zyngier wrote:
> Move all the cp15 registers over to their AArch64 counterpart.
> This requires the annotation of a few of them (such as the usual
> DFAR/IFAR vs FAR_EL1), and a new helper that generates mask/shift
> pairs for the various configurations.

> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 26c7c25f8a6d..137818793a4a 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -138,26 +156,16 @@ static bool access_vm_reg(struct kvm_vcpu *vcpu,
>  			  const struct sys_reg_desc *r)
>  {
>  	bool was_enabled = vcpu_has_cache_enabled(vcpu);
> -	u64 val;
> -	int reg = r->reg;
> +	u64 val, mask, shift;
>  
>  	BUG_ON(!p->is_write);
>  
> -	/* See the 32bit mapping in kvm_host.h */
> -	if (p->is_aarch32)
> -		reg = r->reg / 2;
> +	get_access_mask(r, &mask, &shift);
>  
> -	if (!p->is_aarch32 || !p->is_32bit) {
> -		val = p->regval;
> -	} else {
> -		val = vcpu_read_sys_reg(vcpu, reg);
> -		if (r->reg % 2)
> -			val = (p->regval << 32) | (u64)lower_32_bits(val);
> -		else
> -			val = ((u64)upper_32_bits(val) << 32) |
> -				lower_32_bits(p->regval);
> -	}
> -	vcpu_write_sys_reg(vcpu, val, reg);

> +	val = vcpu_read_sys_reg(vcpu, r->reg);
> +	val &= ~mask;
> +	val |= (p->regval & (mask >> shift)) << shift;
> +	vcpu_write_sys_reg(vcpu, val, r->reg);

I can't tell if the compiler has worked out ithat it can skip the sys_read most of the
time... Won't some of these trap for a nested VHE hypervisor?

| if (~mask) {
|	val = vcpu_read_sys_reg(vcpu, r->reg);
|	val &= ~mask;
| }


But, as the sys_reg arrays already have indirection for this function, why does
access_vm_reg() have to deal with this at all? Its not even needed for all the aarch32
registers...


| { AA32(LO), Op1( 0), CRn(10), CRm( 2), Op2( 0), access_aarch32_alias, NULL, MAIR_EL1 },

Where access_aarch32_alias() does the shift+mask and read_modify-write on the sys_reg?


>  	kvm_toggle_cache(vcpu, was_enabled);
>  	return true;

> @@ -1919,19 +1919,24 @@ static const struct sys_reg_desc cp14_64_regs[] = {
>   */
>  static const struct sys_reg_desc cp15_regs[] = {

> -	{ Op1( 0), CRn( 2), CRm( 0), Op2( 2), access_vm_reg, NULL, c2_TTBCR },

> +	{ Op1( 0), CRn( 2), CRm( 0), Op2( 2), access_vm_reg, NULL, TCR_EL1 },

Don't look now ... TTBRCR2 means this one is a LO/HI job.

(I'm guessing that should be fixed before this series to make the backport easy)


Thanks,

James