Re: [PATCH v2 2/2] KVM: arm64: Commit pending PC adjustemnts before returning to userspace

Alexandru Elisei
2021-05-14T14:09:35Z

Hi Marc,

On 5/14/21 11:40 AM, Marc Zyngier wrote:
> KVM currently updates PC (and the corresponding exception state)
> using a two phase approach: first by setting a set of flags,
> then by converting these flags into a state update when the vcpu
> is about to enter the guest.
>
> However, this creates a disconnect with userspace if the vcpu thread
> returns there with any exception/PC flag set. In this case, the exposed
> context is wrong, as userpsace doesn't have access to these flags

Nitpick: s/userpsace/userspace

> (they aren't architectural). It also means that these flags are
> preserved across a reset, which isn't expected.
>
> To solve this problem, force an explicit synchronisation of the
> exception state on vcpu exit to userspace. As an optimisation
> for nVHE systems, only perform this when there is something pending.
>
> Reported-by: Zenghui Yu <yuzenghui@huawei.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org # 5.11
> ---
> arch/arm64/include/asm/kvm_asm.h | 1 +
> arch/arm64/kvm/arm.c | 10 ++++++++++
> arch/arm64/kvm/hyp/exception.c | 4 ++--
> arch/arm64/kvm/hyp/nvhe/hyp-main.c | 8 ++++++++
> 4 files changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index d5b11037401d..5e9b33cbac51 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -63,6 +63,7 @@
> #define __KVM_HOST_SMCCC_FUNC___pkvm_cpu_set_vector 18
> #define __KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize 19
> #define __KVM_HOST_SMCCC_FUNC___pkvm_mark_hyp 20
> +#define __KVM_HOST_SMCCC_FUNC___kvm_adjust_pc 21
>
> #ifndef __ASSEMBLY__
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 1cb39c0803a4..c4fe2b71f429 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -897,6 +897,16 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>
> kvm_sigset_deactivate(vcpu);
>
> + /*
> + * In the unlikely event that we are returning to userspace
> + * with pending exceptions or PC adjustment, commit these
> + * adjustments in order to give userspace a consistent view of
> + * the vcpu state.
> + */
> + if (unlikely(vcpu->arch.flags & (KVM_ARM64_PENDING_EXCEPTION |
> + KVM_ARM64_INCREMENT_PC)))
> + kvm_call_hyp(__kvm_adjust_pc, vcpu);
> +
> vcpu_put(vcpu);
> return ret;
> }
> diff --git a/arch/arm64/kvm/hyp/exception.c b/arch/arm64/kvm/hyp/exception.c
> index 0812a496725f..11541b94b328 100644
> --- a/arch/arm64/kvm/hyp/exception.c
> +++ b/arch/arm64/kvm/hyp/exception.c
> @@ -331,8 +331,8 @@ static void kvm_inject_exception(struct kvm_vcpu *vcpu)
> }
>
> /*
> - * Adjust the guest PC on entry, depending on flags provided by EL1
> - * for the purpose of emulation (MMIO, sysreg) or exception injection.
> + * Adjust the guest PC (and potentially exception state) depending on
> + * flags provided by the emulation code.
> */
> void __kvm_adjust_pc(struct kvm_vcpu *vcpu)
> {
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index f36420a80474..1632f001f4ed 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -28,6 +28,13 @@ static void handle___kvm_vcpu_run(struct kvm_cpu_context *host_ctxt)
> cpu_reg(host_ctxt, 1) = __kvm_vcpu_run(kern_hyp_va(vcpu));
> }
>
> +static void handle___kvm_adjust_pc(struct kvm_cpu_context *host_ctxt)
> +{
> + DECLARE_REG(struct kvm_vcpu *, vcpu, host_ctxt, 1);
> +
> + __kvm_adjust_pc(kern_hyp_va(vcpu));
> +}
> +
> static void handle___kvm_flush_vm_context(struct kvm_cpu_context *host_ctxt)
> {
> __kvm_flush_vm_context();
> @@ -170,6 +177,7 @@ typedef void (*hcall_t)(struct kvm_cpu_context *);
>
> static const hcall_t host_hcall[] = {
> HANDLE_FUNC(__kvm_vcpu_run),
> + HANDLE_FUNC(__kvm_adjust_pc),
> HANDLE_FUNC(__kvm_flush_vm_context),
> HANDLE_FUNC(__kvm_tlb_flush_vmid_ipa),
> HANDLE_FUNC(__kvm_tlb_flush_vmid),

I'm guessing that the comment mentioned in the cover letter should be in this
patch, right? Or is the changelog from the cover letter stale?

Regardless, I trust your judgement regarding the comment and the patch looks correct:

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>

Thanks,

Alex