Re: [PATCH 18/26] KVM: arm64: Don't use empty structures as CPU reset state

Zenghui Yu
2020-04-24T04:08:12Z

Hi Marc,

On 2020/4/22 20:00, Marc Zyngier wrote:
> Keeping empty structure as the vcpu state initializer is slightly
> wasteful: we only want to set pstate, and zero everything else.
> Just do that.
>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
> arch/arm64/kvm/reset.c | 20 +++++++++-----------
> 1 file changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> index 241db35a7ef4f..895d7d9ad1866 100644
> --- a/arch/arm64/kvm/reset.c
> +++ b/arch/arm64/kvm/reset.c
> @@ -37,15 +37,11 @@ static u32 kvm_ipa_limit;
> /*
> * ARMv8 Reset Values
> */
> -static const struct kvm_regs default_regs_reset = {
> - .regs.pstate = (PSR_MODE_EL1h | PSR_A_BIT | PSR_I_BIT |
> - PSR_F_BIT | PSR_D_BIT),
> -};
> +#define VCPU_RESET_PSTATE_EL1 (PSR_MODE_EL1h | PSR_A_BIT | PSR_I_BIT | \
> + PSR_F_BIT | PSR_D_BIT)
>
> -static const struct kvm_regs default_regs_reset32 = {
> - .regs.pstate = (PSR_AA32_MODE_SVC | PSR_AA32_A_BIT |
> - PSR_AA32_I_BIT | PSR_AA32_F_BIT),
> -};
> +#define VCPU_RESET_PSTATE_SVC (PSR_AA32_MODE_SVC | PSR_AA32_A_BIT | \
> + PSR_AA32_I_BIT | PSR_AA32_F_BIT)
>
> static bool cpu_has_32bit_el1(void)
> {
> @@ -261,6 +257,7 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
> const struct kvm_regs *cpu_reset;
> int ret = -EINVAL;
> bool loaded;
> + u32 pstate;
>
> /* Reset PMU outside of the non-preemptible section */
> kvm_pmu_vcpu_reset(vcpu);
> @@ -291,16 +288,17 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
> if (test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features)) {
> if (!cpu_has_32bit_el1())
> goto out;
> - cpu_reset = &default_regs_reset32;
> + pstate = VCPU_RESET_PSTATE_SVC;
> } else {
> - cpu_reset = &default_regs_reset;
> + pstate = VCPU_RESET_PSTATE_EL1;
> }
>
> break;
> }
>
> /* Reset core registers */
> - memcpy(vcpu_gp_regs(vcpu), cpu_reset, sizeof(*cpu_reset));
> + memset(vcpu_gp_regs(vcpu), 0, sizeof(*cpu_reset));

Be careful that we can *not* use 'sizeof(*cpu_reset)' here anymore. As
you're going to refactor the layout of the core registers whilst keeping
the kvm_regs API unchanged. Resetting the whole kvm_regs will go
corrupting some affected registers and make them temporarily invalid.
The bad thing will show up after you start moving ELR_EL1 around,
specifically in patch #20...

And the first victim is ... MPIDR_EL1 (the first one in sys_regs array).
Now you know how this was spotted ;-) I think this should be the root
cause of what Zengtao had previously reported [*].

If these registers are all expected to be reset to architecturally
UNKNOWN values, I think we can just drop this memset(), though haven't
check with the ARM ARM carefully.

Thanks,
Zenghui

[*]
https://lore.kernel.org/kvmarm/f55386a9-8eaa-944f-453d-9c3c4abee5fb@arm.com/T/#mc6c7268755f5cdaff7a23c34e6e16ea36bcfbe22

> + vcpu_gp_regs(vcpu)->regs.pstate = pstate;
>
> /* Reset system registers */
> kvm_reset_sys_regs(vcpu);