From: Vincent Donnefort <vdonnefort@google.com>
To: tabba@google.com
Cc: Marc Zyngier <maz@kernel.org>, Oliver Upton <oupton@kernel.org>,
Will Deacon <will@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Quentin Perret <qperret@google.com>,
Sebastian Ene <sebastianene@google.com>,
Per Larsen <perlarsen@google.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Zenghui Yu <yuzenghui@huawei.com>,
Joey Gouly <joey.gouly@arm.com>,
Steffen Eiden <seiden@linux.ibm.com>,
Mark Rutland <mark.rutland@arm.com>,
Jonathan Cameron <jonathan.cameron@huawei.com>,
Hyunwoo Kim <imv4bel@gmail.com>,
linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 11/11] KVM: arm64: Implement lazy vCPU state sync for non-protected guests
Date: Mon, 15 Jun 2026 17:25:20 +0100 [thread overview]
Message-ID: <ajAncPp3nOGcWD1U@google.com> (raw)
In-Reply-To: <20260612065925.755562-12-tabba@google.com>
On Fri, Jun 12, 2026 at 07:59:25AM +0100, tabba@google.com wrote:
> pKVM copies a non-protected guest's register context between the host
> and the hypervisor on every world switch, even when the host never
> inspects it. Defer the copy: on entry, flush the host context into the
> hyp vCPU only when the host marked it dirty (PKVM_HOST_STATE_DIRTY); on
> exit, leave it in the hyp vCPU and copy it back only when the host needs
> it, via a __pkvm_vcpu_sync_state hypercall on trap handling or at vcpu
> put. A protected guest's context is copied as before, since lazy sync
> only helps where the host is trusted to see the guest's registers.
>
> The PC is the exception: it is copied back on every exit so the
> kvm_exit tracepoint reports the guest's real exit PC rather than the
> value left by the previous sync.
>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
> arch/arm64/include/asm/kvm_asm.h | 1 +
> arch/arm64/include/asm/kvm_host.h | 2 +
> arch/arm64/kvm/arm.c | 7 +++
> arch/arm64/kvm/handle_exit.c | 22 ++++++++
> arch/arm64/kvm/hyp/nvhe/hyp-main.c | 88 ++++++++++++++++++++++++++++--
> 5 files changed, 115 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index 043495f7fc78..6e1135b3ded4 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -113,6 +113,7 @@ enum __kvm_host_smccc_func {
> __KVM_HOST_SMCCC_FUNC___pkvm_finalize_teardown_vm,
> __KVM_HOST_SMCCC_FUNC___pkvm_vcpu_load,
> __KVM_HOST_SMCCC_FUNC___pkvm_vcpu_put,
> + __KVM_HOST_SMCCC_FUNC___pkvm_vcpu_sync_state,
> __KVM_HOST_SMCCC_FUNC___pkvm_tlb_flush_vmid,
>
> MARKER(__KVM_HOST_SMCCC_FUNC_MAX)
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index a49042bfa801..1ef660774adc 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -1113,6 +1113,8 @@ struct kvm_vcpu_arch {
> /* SError pending for nested guest */
> #define NESTED_SERROR_PENDING __vcpu_single_flag(sflags, BIT(8))
>
> +/* pKVM host vcpu state is dirty, needs resync (nVHE-only) */
nit: with hVHE, I guess we can just drop that nVHE-only?
> +#define PKVM_HOST_STATE_DIRTY __vcpu_single_flag(iflags, BIT(4))
>
> /* Pointer to the vcpu's SVE FFR for sve_{save,load}_state() */
> #define vcpu_sve_pffr(vcpu) (kern_hyp_va((vcpu)->arch.sve_state) + \
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index c9f36932c980..a5c54e37778b 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -734,6 +734,10 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> if (is_protected_kvm_enabled()) {
> kvm_call_hyp(__vgic_v3_save_aprs, &vcpu->arch.vgic_cpu.vgic_v3);
> kvm_call_hyp_nvhe(__pkvm_vcpu_put);
> +
> + /* __pkvm_vcpu_put implies a sync of the state */
> + if (!kvm_vm_is_protected(vcpu->kvm))
> + vcpu_set_flag(vcpu, PKVM_HOST_STATE_DIRTY);
> }
>
> kvm_vcpu_put_debug(vcpu);
> @@ -961,6 +965,9 @@ int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
> return ret;
>
> if (is_protected_kvm_enabled()) {
> + /* Start with the vcpu in a dirty state */
> + if (!kvm_vm_is_protected(vcpu->kvm))
> + vcpu_set_flag(vcpu, PKVM_HOST_STATE_DIRTY);
> ret = pkvm_create_hyp_vm(kvm);
> if (ret)
> return ret;
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 54aedf93c78b..dccc3786548b 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -422,6 +422,21 @@ static int handle_trap_exceptions(struct kvm_vcpu *vcpu)
> {
> int handled;
>
> + /*
> + * If we run a non-protected VM when protection is enabled
> + * system-wide, resync the state from the hypervisor and mark
> + * it as dirty on the host side if it wasn't dirty already
> + * (which could happen if preemption has taken place).
> + */
> + if (is_protected_kvm_enabled() && !kvm_vm_is_protected(vcpu->kvm)) {
> + preempt_disable();
nit: since we are introducing guard() with that series, this one could be
guard(preempt)().
> + if (!(vcpu_get_flag(vcpu, PKVM_HOST_STATE_DIRTY))) {
> + kvm_call_hyp_nvhe(__pkvm_vcpu_sync_state);
> + vcpu_set_flag(vcpu, PKVM_HOST_STATE_DIRTY);
> + }
> + preempt_enable();
> + }
> +
> /*
> * See ARM ARM B1.14.1: "Hyp traps on instructions
> * that fail their condition code check"
> @@ -489,6 +504,13 @@ int handle_exit(struct kvm_vcpu *vcpu, int exception_index)
> /* For exit types that need handling before we can be preempted */
> void handle_exit_early(struct kvm_vcpu *vcpu, int exception_index)
> {
> + /*
> + * We just exited, so the state is clean from a hypervisor
> + * perspective.
> + */
> + if (is_protected_kvm_enabled())
> + vcpu_clear_flag(vcpu, PKVM_HOST_STATE_DIRTY);
> +
> if (ARM_SERROR_PENDING(exception_index)) {
> if (this_cpu_has_cap(ARM64_HAS_RAS_EXTN)) {
> u64 disr = kvm_vcpu_get_disr(vcpu);
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index 23e644c24a03..02383b372258 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -139,6 +139,49 @@ static void sync_hyp_vgic_state(struct pkvm_hyp_vcpu *hyp_vcpu)
> host_cpu_if->vgic_lr[i] = hyp_cpu_if->vgic_lr[i];
> }
>
> +
> +static void __copy_vcpu_state(const struct kvm_vcpu *from_vcpu,
> + struct kvm_vcpu *to_vcpu)
> +{
> + int i;
> +
> + to_vcpu->arch.ctxt.regs = from_vcpu->arch.ctxt.regs;
> + to_vcpu->arch.ctxt.spsr_abt = from_vcpu->arch.ctxt.spsr_abt;
> + to_vcpu->arch.ctxt.spsr_und = from_vcpu->arch.ctxt.spsr_und;
> + to_vcpu->arch.ctxt.spsr_irq = from_vcpu->arch.ctxt.spsr_irq;
> + to_vcpu->arch.ctxt.spsr_fiq = from_vcpu->arch.ctxt.spsr_fiq;
> + to_vcpu->arch.ctxt.fp_regs = from_vcpu->arch.ctxt.fp_regs;
> +
> + /*
> + * Copy the sysregs, but don't mess with the timer state which
> + * is directly handled by EL1 and is expected to be preserved.
> + * enum vcpu_sysreg is sparse: VNCR-mapped registers take values
> + * derived from their VNCR page offset, so the timer registers do
> + * not form a contiguous numeric range and must be skipped by name.
> + */
> + for (i = 1; i < NR_SYS_REGS; i++) {
> + switch (i) {
> + case CNTVOFF_EL2:
> + case CNTV_CVAL_EL0:
> + case CNTV_CTL_EL0:
> + case CNTP_CVAL_EL0:
> + case CNTP_CTL_EL0:
> + continue;
> + }
> + to_vcpu->arch.ctxt.sys_regs[i] = from_vcpu->arch.ctxt.sys_regs[i];
> + }
> +}
> +
> +static void __sync_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> +{
> + __copy_vcpu_state(&hyp_vcpu->vcpu, hyp_vcpu->host_vcpu);
> +}
> +
> +static void __flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> +{
> + __copy_vcpu_state(hyp_vcpu->host_vcpu, &hyp_vcpu->vcpu);
> +}
nit: Could that be flush/sync_hyp_vcpu_state? as everything this is called
"state" and we already have flush_debug_state() below ?
> +
> static void flush_debug_state(struct pkvm_hyp_vcpu *hyp_vcpu)
> {
> struct kvm_vcpu *host_vcpu = hyp_vcpu->host_vcpu;
> @@ -168,7 +211,17 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> fpsimd_sve_flush();
> flush_debug_state(hyp_vcpu);
>
> - hyp_vcpu->vcpu.arch.ctxt = host_vcpu->arch.ctxt;
> + /*
> + * If we deal with a non-protected guest and the state is potentially
> + * dirty (from a host perspective), copy the state back into the hyp
> + * vcpu.
> + */
> + if (!pkvm_hyp_vcpu_is_protected(hyp_vcpu)) {
> + if (vcpu_get_flag(host_vcpu, PKVM_HOST_STATE_DIRTY))
> + __flush_hyp_vcpu(hyp_vcpu);
> + } else {
> + hyp_vcpu->vcpu.arch.ctxt = host_vcpu->arch.ctxt;
> + }
>
> hyp_vcpu->vcpu.arch.mdcr_el2 = host_vcpu->arch.mdcr_el2;
> hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWI | HCR_TWE);
> @@ -191,9 +244,11 @@ static void sync_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> fpsimd_sve_sync(&hyp_vcpu->vcpu);
> sync_debug_state(hyp_vcpu);
>
> - host_vcpu->arch.ctxt = hyp_vcpu->vcpu.arch.ctxt;
> -
> - host_vcpu->arch.hcr_el2 = hyp_vcpu->vcpu.arch.hcr_el2;
> + if (pkvm_hyp_vcpu_is_protected(hyp_vcpu))
> + host_vcpu->arch.ctxt = hyp_vcpu->vcpu.arch.ctxt;
> + else
> + /* Keep the PC current for the kvm_exit tracepoint (lazy ctxt sync). */
> + host_vcpu->arch.ctxt.regs.pc = hyp_vcpu->vcpu.arch.ctxt.regs.pc;
>
> host_vcpu->arch.fault = hyp_vcpu->vcpu.arch.fault;
>
> @@ -227,8 +282,30 @@ static void handle___pkvm_vcpu_put(struct kvm_cpu_context *host_ctxt)
> {
> struct pkvm_hyp_vcpu *hyp_vcpu = pkvm_get_loaded_hyp_vcpu();
>
> - if (hyp_vcpu)
> + if (hyp_vcpu) {
> + struct kvm_vcpu *host_vcpu = hyp_vcpu->host_vcpu;
> +
> + if (!pkvm_hyp_vcpu_is_protected(hyp_vcpu) &&
> + !vcpu_get_flag(host_vcpu, PKVM_HOST_STATE_DIRTY)) {
> + __sync_hyp_vcpu(hyp_vcpu);
> + }
> +
> pkvm_put_hyp_vcpu(hyp_vcpu);
> + }
> +}
> +
> +static void handle___pkvm_vcpu_sync_state(struct kvm_cpu_context *host_ctxt)
> +{
> + struct pkvm_hyp_vcpu *hyp_vcpu;
> +
> + if (!is_protected_kvm_enabled())
> + return;
Since "KVM: arm64: Remove is_protected_kvm_enabled() checks from hypercalls" we
got rid of those is_protected_kvm_enabled() for pKVM-only HVCs. (also, it is
declared in the pKVM-only section of the HVCs)
> +
> + hyp_vcpu = pkvm_get_loaded_hyp_vcpu();
> + if (!hyp_vcpu || pkvm_hyp_vcpu_is_protected(hyp_vcpu))
> + return;
> +
> + __sync_hyp_vcpu(hyp_vcpu);
> }
>
> static struct kvm_vcpu *__get_host_hyp_vcpus(struct kvm_vcpu *arg,
> @@ -859,6 +936,7 @@ static const hcall_t host_hcall[] = {
> HANDLE_FUNC(__pkvm_finalize_teardown_vm),
> HANDLE_FUNC(__pkvm_vcpu_load),
> HANDLE_FUNC(__pkvm_vcpu_put),
> + HANDLE_FUNC(__pkvm_vcpu_sync_state),
> HANDLE_FUNC(__pkvm_tlb_flush_vmid),
> };
>
> --
> 2.54.0.1136.gdb2ca164c4-goog
>
next prev parent reply other threads:[~2026-06-15 16:25 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-06-12 6:59 [PATCH v1 00/11] KVM: arm64: Rework pKVM vCPU state synchronisation tabba
2026-06-12 6:59 ` [PATCH v1 01/11] KVM: arm64: Add scoped resource management (guard) for hyp_spinlock tabba
2026-06-12 6:59 ` [PATCH v1 02/11] KVM: arm64: Use guard(hyp_spinlock) in pKVM hypervisor code tabba
2026-06-15 12:53 ` Vincent Donnefort
2026-06-15 13:11 ` Fuad Tabba
2026-06-12 6:59 ` [PATCH v1 03/11] KVM: arm64: Use guard()/scoped_guard() in arm64 KVM EL1 code tabba
2026-06-15 12:59 ` Vincent Donnefort
2026-06-15 13:17 ` Fuad Tabba
2026-06-12 6:59 ` [PATCH v1 04/11] KVM: arm64: Extract MPIDR computation into a shared header tabba
2026-06-12 6:59 ` [PATCH v1 05/11] KVM: arm64: Make vcpu_{read,write}_sys_reg available to HYP code tabba
2026-06-15 13:11 ` Vincent Donnefort
2026-06-15 13:29 ` Fuad Tabba
2026-06-12 6:59 ` [PATCH v1 06/11] KVM: arm64: Factor out reusable vCPU reset helpers tabba
2026-06-15 13:16 ` Vincent Donnefort
2026-06-15 13:45 ` Fuad Tabba
2026-06-12 6:59 ` [PATCH v1 07/11] KVM: arm64: Move PSCI helper functions to a shared header tabba
2026-06-12 6:59 ` [PATCH v1 08/11] KVM: arm64: Add host and hypervisor vCPU lookup primitives tabba
2026-06-12 6:59 ` [PATCH v1 09/11] KVM: arm64: Minimise EL2's exposure of host VGIC state during world switch tabba
2026-06-12 6:59 ` [PATCH v1 10/11] KVM: arm64: Add primitives to flush/sync the VGIC state at EL2 tabba
2026-06-12 6:59 ` [PATCH v1 11/11] KVM: arm64: Implement lazy vCPU state sync for non-protected guests tabba
2026-06-15 16:25 ` Vincent Donnefort [this message]
2026-06-15 16:44 ` Fuad Tabba
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