[RFC PATCH v3 02/29] KVM: arm64: Save ID registers' sanitized value per vCPU

[Marc Zyngier]

On Wed, 17 Nov 2021 06:43:32 +0000,
Reiji Watanabe <reijiw at google.com> wrote:
> 
> Extend sys_regs[] of kvm_cpu_context for ID registers and save ID
> registers' sanitized value in the array for the vCPU at the first
> vCPU reset. Use the saved ones when ID registers are read by
> userspace (via KVM_GET_ONE_REG) or the guest.
> 
> Signed-off-by: Reiji Watanabe <reijiw at google.com>
> ---
>  arch/arm64/include/asm/kvm_host.h | 10 +++++++
>  arch/arm64/kvm/sys_regs.c         | 43 +++++++++++++++++++------------
>  2 files changed, 37 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index edbe2cb21947..72db73c79403 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -146,6 +146,14 @@ struct kvm_vcpu_fault_info {
>  	u64 disr_el1;		/* Deferred [SError] Status Register */
>  };
>  
> +/*
> + * (Op0, Op1, CRn, CRm, Op2) of ID registers is (3, 0, 0, crm, op2),
> + * where 0<=crm<8, 0<=op2<8.
> + */
> +#define KVM_ARM_ID_REG_MAX_NUM 64
> +#define IDREG_IDX(id)		((sys_reg_CRm(id) << 3) | sys_reg_Op2(id))
> +#define IDREG_SYS_IDX(id)	(ID_REG_BASE + IDREG_IDX(id))
> +
>  enum vcpu_sysreg {
>  	__INVALID_SYSREG__,   /* 0 is reserved as an invalid value */
>  	MPIDR_EL1,	/* MultiProcessor Affinity Register */
> @@ -210,6 +218,8 @@ enum vcpu_sysreg {
>  	CNTP_CVAL_EL0,
>  	CNTP_CTL_EL0,
>  
> +	ID_REG_BASE,
> +	ID_REG_END = ID_REG_BASE + KVM_ARM_ID_REG_MAX_NUM - 1,

It is rather unclear to me why we want these registers to be
replicated on a per-CPU basis. Yes, this fits the architecture, but
that's also a total waste of memory if you have more than a single
CPU, because we make a point in only exposing homogeneous properties
to the VM (I don't think anyone intends to support vcpu asymmetry in a
VM, and 64 registers per vcpu is not an insignificant memory usage).

If there are no reasons for this to be per-CPU, please move it to be
global to the VM. This also mean that once a vcpu has reset, it
shouldn't be possible to affect the registers. This shouldn't affect
the userspace API though.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.