Re: [PATCH v2 24/28] arm64/sve: KVM: Hide SVE from CPU features exposed to guests

["Alex Bennée" <alex.bennee@linaro.org>]

Dave Martin <Dave.Martin@arm.com> writes:

> 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>
> Cc: Marc Zyngier <marc.zyngier@arm.com>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

>
> ---
>
> Changes since v1
> ----------------
>
> Requested by Marc Zyngier:
>
> * Use pr_err() instead inventing "kvm_info_once" ad-hoc.
> ---
>  arch/arm64/kvm/sys_regs.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index b1f7552..a0ee9b0 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -23,6 +23,7 @@
>  #include <linux/bsearch.h>
>  #include <linux/kvm_host.h>
>  #include <linux/mm.h>
> +#include <linux/printk.h>
>  #include <linux/uaccess.h>
>
>  #include <asm/cacheflush.h>
> @@ -897,8 +898,17 @@ 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) {
> +		if (val & (0xfUL << ID_AA64PFR0_SVE_SHIFT))
> +			pr_err_once("kvm [%i]: SVE unsupported for guests, suppressing\n",
> +				    task_pid_nr(current));
> +
> +		val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> +	}
> +
> +	return val;
>  }
>
>  /* cpufeature ID register access trap handlers */