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[34.77.69.137]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49269005626sm54095525e9.10.2026.06.26.00.26.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 26 Jun 2026 00:26:33 -0700 (PDT) Date: Fri, 26 Jun 2026 08:26:29 +0100 From: Vincent Donnefort To: Fuad Tabba Cc: Marc Zyngier , Oliver Upton , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Catalin Marinas , Will Deacon , Joey Gouly , Steffen Eiden , Suzuki K Poulose , Zenghui Yu , Quentin Perret , Sebastian Ene , Hyunwoo Kim , Fuad Tabba Subject: Re: [PATCH v3 8/8] KVM: arm64: Implement lazy vCPU state sync for non-protected guests Message-ID: References: <20260626070408.3420953-1-fuad.tabba@linux.dev> <20260626070408.3420953-9-fuad.tabba@linux.dev> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260626070408.3420953-9-fuad.tabba@linux.dev> On Fri, Jun 26, 2026 at 08:04:08AM +0100, Fuad Tabba 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 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. > > PC and PSTATE are the exception: they are copied back on every exit so > the kvm_exit tracepoint reports the guest's real exit PC, and the run > loop's vcpu_mode_is_bad_32bit() and SError-masking checks evaluate the > guest's current PSTATE rather than the value left by the previous sync. > > The host needs the full context when it is about to read it (trap > handling) or write it (the SError injection that writes ESR_EL1). Sync > both from handle_exit_early(), which runs non-preemptible so the loaded > hyp vCPU is stable without a preempt guard. > > Signed-off-by: Fuad Tabba Reviewed-by: Vincent Donnefort > --- > 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 | 23 ++++++++ > arch/arm64/kvm/hyp/nvhe/hyp-main.c | 86 ++++++++++++++++++++++++++++-- > 5 files changed, 114 insertions(+), 5 deletions(-) > > diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h > index 043495f7fc78b..6e1135b3ded44 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 2faa60df847d2..caa39ee5125f2 100644 > --- a/arch/arm64/include/asm/kvm_host.h > +++ b/arch/arm64/include/asm/kvm_host.h > @@ -1068,6 +1068,8 @@ struct kvm_vcpu_arch { > #define INCREMENT_PC __vcpu_single_flag(iflags, BIT(1)) > /* Target EL/MODE (not a single flag, but let's abuse the macro) */ > #define EXCEPT_MASK __vcpu_single_flag(iflags, GENMASK(3, 1)) > +/* Host-set: the hyp flushes the non-protected vCPU state in on entry */ > +#define PKVM_HOST_STATE_DIRTY __vcpu_single_flag(iflags, BIT(4)) > > /* Helpers to encode exceptions with minimum fuss */ > #define __EXCEPT_MASK_VAL unpack_vcpu_flag(EXCEPT_MASK) > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > index 3732ee9eb0d4e..4e89558d80278 100644 > --- a/arch/arm64/kvm/arm.c > +++ b/arch/arm64/kvm/arm.c > @@ -733,6 +733,10 @@ 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); > @@ -964,6 +968,9 @@ 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 54aedf93c78b6..29108e5c0206e 100644 > --- a/arch/arm64/kvm/handle_exit.c > +++ b/arch/arm64/kvm/handle_exit.c > @@ -486,9 +486,32 @@ int handle_exit(struct kvm_vcpu *vcpu, int exception_index) > } > } > > +static void handle_exit_pkvm_state(struct kvm_vcpu *vcpu, int exception_index) > +{ > + int exception_code = ARM_EXCEPTION_CODE(exception_index); > + > + if (!is_protected_kvm_enabled() || kvm_vm_is_protected(vcpu->kvm)) > + return; > + > + /* > + * Sync the context back when the host will read (trap) or write > + * (SError) it. Preempt-off here, so the loaded hyp vCPU is stable. > + */ > + if (exception_code == ARM_EXCEPTION_TRAP || > + exception_code == ARM_EXCEPTION_EL1_SERROR || > + ARM_SERROR_PENDING(exception_index)) { > + kvm_call_hyp_nvhe(__pkvm_vcpu_sync_state); > + vcpu_set_flag(vcpu, PKVM_HOST_STATE_DIRTY); > + } else { > + vcpu_clear_flag(vcpu, PKVM_HOST_STATE_DIRTY); > + } > +} > + > /* For exit types that need handling before we can be preempted */ > void handle_exit_early(struct kvm_vcpu *vcpu, int exception_index) > { > + handle_exit_pkvm_state(vcpu, exception_index); > + > 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 0194965930e61..acf53aae4fe43 100644 > --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c > +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c > @@ -141,6 +141,48 @@ 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_state(struct pkvm_hyp_vcpu *hyp_vcpu) > +{ > + __copy_vcpu_state(&hyp_vcpu->vcpu, hyp_vcpu->host_vcpu); > +} > + > +static void flush_hyp_vcpu_state(struct pkvm_hyp_vcpu *hyp_vcpu) > +{ > + __copy_vcpu_state(hyp_vcpu->host_vcpu, &hyp_vcpu->vcpu); > +} > + > static void flush_debug_state(struct pkvm_hyp_vcpu *hyp_vcpu) > { > struct kvm_vcpu *host_vcpu = hyp_vcpu->host_vcpu; > @@ -170,7 +212,17 @@ 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_state(hyp_vcpu); > + } else { > + hyp_vcpu->vcpu.arch.ctxt = host_vcpu->arch.ctxt; > + } > > /* __hyp_running_vcpu must be NULL in a guest context. */ > hyp_vcpu->vcpu.arch.ctxt.__hyp_running_vcpu = NULL; > @@ -201,9 +253,13 @@ 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 PC (tracepoint) and PSTATE (vcpu_mode_is_bad_32bit) current. */ > + host_vcpu->arch.ctxt.regs.pc = hyp_vcpu->vcpu.arch.ctxt.regs.pc; > + host_vcpu->arch.ctxt.regs.pstate = hyp_vcpu->vcpu.arch.ctxt.regs.pstate; > + } > > host_vcpu->arch.fault = hyp_vcpu->vcpu.arch.fault; > > @@ -237,8 +293,27 @@ 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_state(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; > + > + hyp_vcpu = pkvm_get_loaded_hyp_vcpu(); > + if (!hyp_vcpu || pkvm_hyp_vcpu_is_protected(hyp_vcpu)) > + return; > + > + sync_hyp_vcpu_state(hyp_vcpu); > } > > static struct kvm_vcpu *__get_host_hyp_vcpus(struct kvm_vcpu *arg, > @@ -869,6 +944,7 @@ 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), > }; > > -- > 2.39.5 >