Re: [PATCH v4 1/6] KVM: arm64: Correctly treat writes to OSLSR_EL1 as undefined

[Mark Rutland <mark.rutland@arm.com>]

Hi Oliver,

On Tue, Dec 14, 2021 at 05:28:07PM +0000, Oliver Upton wrote:
> Any valid implementation of the architecture should generate an
> undefined exception for writes to a read-only register, such as
> OSLSR_EL1. Nonetheless, the KVM handler actually implements write-ignore
> behavior.
> 
> Align the trap handler for OSLSR_EL1 with hardware behavior. If such a
> write ever traps to EL2, inject an undef into the guest and print a
> warning.

I think this can still be read amibguously, since we don't explicitly state
that writes to OSLSR_EL1 should never trap (and the implications of being
UNDEFINED are subtle). How about:

| Writes to OSLSR_EL1 are UNDEFINED and should never trap from EL1 to EL2, but
| the KVM trap handler for OSLSR_EL1 handlees writes via ignore_write(). This
| is confusing to readers of the code, but shouldn't have any functional impact.
|
| For clarity, use write_to_read_only() rather than ignore_write(). If a trap
| is unexpectedly taken to EL2 in violation of the architecture, this will
| WARN_ONCE() and inject an undef into the guest.

With that:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> Reviewed-by: Reiji Watanabe <reijiw@google.com>
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
>  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 e3ec1a44f94d..11b4212c2036 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -292,7 +292,7 @@ static bool trap_oslsr_el1(struct kvm_vcpu *vcpu,
>  			   const struct sys_reg_desc *r)
>  {
>  	if (p->is_write) {
> -		return ignore_write(vcpu, p);
> +		return write_to_read_only(vcpu, p, r);
>  	} else {
>  		p->regval = (1 << 3);
>  		return true;
> -- 
> 2.34.1.173.g76aa8bc2d0-goog
> 
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