From 49930b1b557ceb698e0ab7f985ea41c99dc8bf22 Mon Sep 17 00:00:00 2001 From: Fuad Tabba Date: Wed, 7 Oct 2026 08:43:41 +0100 Subject: [PATCH] KVM: arm64: timers: Compute an offset-applied CVAL from the current count When the hardware won't apply a timer's offset, KVM programs CVAL + offset and lets it compare that against the raw counter. The compare is unsigned (IsTimerConditionMet, DDI0487M_c J1.4.3.29): condition_met = (UInt(PhysicalCountInt() - offset) - UInt(compare_value)) >= 0; so adding the offset to both sides holds only while both sums wrap or neither does. A guest whose counter is ahead of the host's by more than CVAL has an expired timer that never fires, and one behind it can have a far-future timer fire at once. On a CNTPOFF_EL2 host the first case livelocks: each entry reloads the guest's CVAL, the timer fires, and the exit rewrites it as CVAL + offset before the host can take the interrupt. Without CNTPOFF_EL2 the timer hangs until the vCPU is next loaded, which is what Zenghui reported on a Kunpeng920. Program the value from the current count instead: CVAL + offset while the deadline is ahead, 0 once the timer has expired for the guest, ~0 past the counter's wrap. KVM recomputes it at load, at exit and on a trapped CVAL write, and an expired timer stays expired in between. The value can't be inverted, so with an offset timer_save_state() keeps the memory copy, and kvm_hyp_handle_timer() returns that copy for a guest hypervisor's read of its physical CVAL rather than subtracting the offset back out. Fixes: c605ee245097 ("KVM: arm64: timers: Allow physical offset without CNTPOFF_EL2") Fixes: 9404673293b0 ("KVM: arm64: timers: Correctly handle TGE flip with CNTPOFF_EL2") Fixes: 0bc9a9e85fcf ("KVM: arm64: Work around x1e's CNTVOFF_EL2 bogosity") Reported-by: Zenghui Yu Closes: https://lore.kernel.org/r/460258be-0102-e922-c342-4e87cd94b9e5@huawei.com Cc: stable@vger.kernel.org Signed-off-by: Fuad Tabba Link: https://patch.msgid.link/20261007074343.324269-2-fuad.tabba@linux.dev Signed-off-by: Marc Zyngier --- arch/arm64/kvm/arch_timer.c | 27 ++++++++++++--------------- arch/arm64/kvm/hyp/vhe/switch.c | 11 ++++++----- include/kvm/arm_arch_timer.h | 15 +++++++++++++++ 3 files changed, 33 insertions(+), 20 deletions(-) diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c index 6ac3321f4c57..9278b5383c04 100644 --- a/arch/arm64/kvm/arch_timer.c +++ b/arch/arm64/kvm/arch_timer.c @@ -528,17 +528,11 @@ static void timer_save_state(struct arch_timer_context *ctx) goto out; switch (index) { - u64 cval; - case TIMER_VTIMER: case TIMER_HVTIMER: timer_set_ctl(ctx, read_sysreg_el0(SYS_CNTV_CTL)); - cval = read_sysreg_el0(SYS_CNTV_CVAL); - - if (has_broken_cntvoff()) - cval -= timer_get_offset(ctx); - - timer_set_cval(ctx, cval); + if (!has_broken_cntvoff() || !timer_get_offset(ctx)) + timer_set_cval(ctx, read_sysreg_el0(SYS_CNTV_CVAL)); /* Disable the timer */ write_sysreg_el0(0, SYS_CNTV_CTL); @@ -564,11 +558,12 @@ static void timer_save_state(struct arch_timer_context *ctx) case TIMER_PTIMER: case TIMER_HPTIMER: timer_set_ctl(ctx, read_sysreg_el0(SYS_CNTP_CTL)); - cval = read_sysreg_el0(SYS_CNTP_CVAL); - - cval -= timer_get_offset(ctx); - - timer_set_cval(ctx, cval); + /* + * With an offset, memory already holds the guest's CVAL (the + * trap handler or __deactivate_traps() wrote it). + */ + if (!timer_get_offset(ctx)) + timer_set_cval(ctx, read_sysreg_el0(SYS_CNTP_CVAL)); /* Disable the timer */ write_sysreg_el0(0, SYS_CNTP_CTL); @@ -647,7 +642,8 @@ static void timer_restore_state(struct arch_timer_context *ctx) offset = timer_get_offset(ctx); if (has_broken_cntvoff()) { set_cntvoff(0); - cval += offset; + if (offset) + cval = timer_apply_offset(cval, offset, kvm_phys_timer_read()); } else { set_cntvoff(offset); } @@ -660,7 +656,8 @@ static void timer_restore_state(struct arch_timer_context *ctx) cval = timer_get_cval(ctx); offset = timer_get_offset(ctx); set_cntpoff(offset); - cval += offset; + if (offset) + cval = timer_apply_offset(cval, offset, kvm_phys_timer_read()); write_sysreg_el0(cval, SYS_CNTP_CVAL); isb(); write_sysreg_el0(timer_get_ctl(ctx), SYS_CNTP_CTL); diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c index 7875911c0506..14aada311bac 100644 --- a/arch/arm64/kvm/hyp/vhe/switch.c +++ b/arch/arm64/kvm/hyp/vhe/switch.c @@ -175,7 +175,8 @@ static void __deactivate_traps(struct kvm_vcpu *vcpu) offset = read_sysreg_s(SYS_CNTPOFF_EL2); if (map.direct_ptimer && offset) { - write_sysreg_el0(val + offset, SYS_CNTP_CVAL); + val = timer_apply_offset(val, offset, arch_timer_read_cntpct_el0()); + write_sysreg_el0(val, SYS_CNTP_CVAL); isb(); } } @@ -296,10 +297,10 @@ static bool kvm_hyp_handle_timer(struct kvm_vcpu *vcpu, u64 *exit_code) break; case SYS_CNTP_CVAL_EL0: if (vcpu_el2_e2h_is_set(vcpu)) { - val = read_sysreg_el0(SYS_CNTP_CVAL); - - if (!has_cntpoff()) - val -= timer_get_offset(vcpu_hptimer(vcpu)); + if (!has_cntpoff() && timer_get_offset(vcpu_hptimer(vcpu))) + val = __vcpu_sys_reg(vcpu, CNTHP_CVAL_EL2); + else + val = read_sysreg_el0(SYS_CNTP_CVAL); } else { val = __vcpu_sys_reg(vcpu, CNTP_CVAL_EL0); } diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h index bc6f2fdd7ad3..80f96ea59f1a 100644 --- a/include/kvm/arm_arch_timer.h +++ b/include/kvm/arm_arch_timer.h @@ -195,4 +195,19 @@ static inline void timer_set_offset(struct arch_timer_context *ctxt, u64 offset) WRITE_ONCE(*ctxt->offset.vm_offset, offset); } +/* + * CVAL to program when the hardware won't apply the timer's offset, so that + * its compare against the raw counter matches the guest's at 'now'. An + * expired timer stays expired; one past the wrap never fires. + */ +static inline u64 timer_apply_offset(u64 cval, u64 offset, u64 now) +{ + u64 hw = cval + offset; + + if (now - offset >= cval) + return 0; + + return hw > now ? hw : U64_MAX; +} + #endif