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KVM: arm64: timers: Compute an offset-applied CVAL from the current count

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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: c605ee2450 ("KVM: arm64: timers: Allow physical offset without CNTPOFF_EL2")
Fixes: 9404673293 ("KVM: arm64: timers: Correctly handle TGE flip with CNTPOFF_EL2")
Fixes: 0bc9a9e85f ("KVM: arm64: Work around x1e's CNTVOFF_EL2 bogosity")
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Closes: https://lore.kernel.org/r/460258be-0102-e922-c342-4e87cd94b9e5@huawei.com
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20261007074343.324269-2-fuad.tabba@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>
This commit is contained in:
Fuad Tabba 2026-10-07 08:43:41 +01:00 committed by Marc Zyngier
parent 517cdec3d4
commit 49930b1b55
3 changed files with 33 additions and 20 deletions
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27
arch/arm64/kvm/arch_timer.c
View File

@ -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);

11
arch/arm64/kvm/hyp/vhe/switch.c
View File

@ -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);
}

15
include/kvm/arm_arch_timer.h
View File

@ -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