Re: [PATCH 23/27] arm64/sve: KVM: Hide SVE from CPU features exposed to guests

Marc Zyngier
2017-08-15T16:37:55Z

On 09/08/17 13:05, Dave Martin wrote:
> KVM guests cannot currently use SVE, because SVE is always
> configured to trap to EL2.
>
> However, a guest that sees SVE reported as present in
> ID_AA64PFR0_EL1 may legitimately expect that SVE works and try to
> use it. Instead of working, the guest will receive an injected
> undef exception, which may cause the guest to oops or go into a
> spin.
>
> To avoid misleading the guest into believing that SVE will work,
> this patch masks out the SVE field from ID_AA64PFR0_EL1 when a
> guest attempts to read this register. No support is explicitly
> added for ID_AA64ZFR0_EL1 either, so that is still emulated as
> reading as zero, which is consistent with SVE not being
> implemented.
>
> This is a temporary measure, and will be removed in a later series
> when full KVM support for SVE is implemented.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> ---
> arch/arm64/kvm/sys_regs.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 6583dd7..9e8c54e 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -897,8 +897,20 @@ static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
> {
> u32 id = sys_reg((u32)r->Op0, (u32)r->Op1,
> (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
> + u64 val = raz ? 0 : read_sanitised_ftr_reg(id);
>
> - return raz ? 0 : read_sanitised_ftr_reg(id);
> + if (id == SYS_ID_AA64PFR0_EL1) {
> + static bool printed;
> +
> + if ((val & (0xfUL << ID_AA64PFR0_SVE_SHIFT)) && !printed) {
> + kvm_info("SVE unsupported for guests, suppressing\n");
> + printed = true;
> + }

Ideally, this should be a vcpu_unimpl_once(). But:
- it doesn't exist
- vcpu_unimpl looks hopelessly x86 specific

How about turning it into a pr_err_once() instead?

> +
> + val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> + }
> +
> + return val;
> }
>
> /* cpufeature ID register access trap handlers */
>

Thanks,

M.
--
Jazz is not dead. It just smells funny...