Re: [RFC PATCH v1 1/2] KVM: arm64: Allow BT field in ID_AA64PFR1_EL1 writable

[Oliver Upton <oliver.upton@linux.dev>]

On Tue, Jun 11, 2024 at 10:35:51PM -0400, Shaoqin Huang wrote:
> When migrating from MtCollins to AmpereOne, the BT field value in
> ID_AA64PFR1_EL1 register is different and not writable. This causes
> the migration to fail.
> 
> The BT field means Branch Target Identification mechanism support in
> AArch64 state. The value 0 means BT is not implemented, the value 1
> means BT is implemented.
> 
> On MtCollins(Migration Src), the BT value is 0.
> On AmpereOne(Migration Dst), the BT value is 1.
> 
> As it defined in the ftr_id_aa64dfr0, the samller value is safe. So if

typo: smaller

> we make the BT field writable, on the AmpereOne(Migration Dst) the BT
> field will be overrided with value 0.
> 
> Signed-off-by: Shaoqin Huang <shahuang@redhat.com>
> ---
> But there is a question, the ARM DDI mentions from Armv8.5, the only
> permitted value is 0b01. Do you guys know if there are any consequence
> if the userspace write value 0b0 into this field? Or we should restrict
> that at some level, like in VMM or kernel level?

There's no directly visible attribute in the CPU registers to determine
what level of the architecture the implementation supports, and I
don't really want KVM to go about policing this.

The general guidance for ID register fields is that we allow userspace
to select a subset of CPU features supported by KVM / the
implementation, which in this case would include the _NI encoding for
the field. This has been slightly opinionated so far, leaving features
that userspace selects via a separate mechanism (e.g. KVM_ARM_VCPU_INIT)
read only.

Userspace can (and should) come up with its own heuristics for
determining the feature set for the vCPU.

> ---
>  arch/arm64/kvm/sys_regs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index c9f4f387155f..8e0ea62e14e1 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2292,7 +2292,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  		   ID_AA64PFR0_EL1_GIC |
>  		   ID_AA64PFR0_EL1_AdvSIMD |
>  		   ID_AA64PFR0_EL1_FP), },
> -	ID_SANITISED(ID_AA64PFR1_EL1),
> +	ID_WRITABLE(SYS_ID_AA64PFR1_EL1, ID_AA64PFR1_EL1_BT),

This doesn't compile. The macro prefixes "SYS_" to the register name.

-- 
Thanks,
Oliver