On Wed, May 14, 2025, Yuntao Liu wrote:
> Call Trace:
> <TASK>
> kvm_get_time_scale arch/x86/kvm/x86.c:2458 [inline]
> kvm_guest_time_update+0x926/0xb00 arch/x86/kvm/x86.c:3268
> vcpu_enter_guest.constprop.0+0x1e70/0x3cf0 arch/x86/kvm/x86.c:10678
> vcpu_run+0x129/0x8d0 arch/x86/kvm/x86.c:11126
> kvm_arch_vcpu_ioctl_run+0x37a/0x13d0 arch/x86/kvm/x86.c:11352
> kvm_vcpu_ioctl+0x56b/0xe60 virt/kvm/kvm_main.c:4188
> vfs_ioctl fs/ioctl.c:51 [inline]
> __do_sys_ioctl fs/ioctl.c:871 [inline]
> __se_sys_ioctl+0x12d/0x190 fs/ioctl.c:857
> do_syscall_x64 arch/x86/entry/common.c:51 [inline]
> do_syscall_64+0x59/0x110 arch/x86/entry/common.c:81
> entry_SYSCALL_64_after_hwframe+0x78/0xe2
>
> ioctl$KVM_SET_TSC_KHZ(r2, 0xaea2, 0x1)
> user_tsc_khz = 0x1
> |
> kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz)
> |
> ioctl$KVM_RUN(r2, 0xae80, 0x0)
> |
> ...
> kvm_guest_time_update(struct kvm_vcpu *v)
> |
> if (kvm_caps.has_tsc_control)
> tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz,
> v->arch.l1_tsc_scaling_ratio);
> |
> kvm_scale_tsc(u64 tsc, u64 ratio)
> |
> __scale_tsc(u64 ratio, u64 tsc)
> ratio=122380531, tsc=2299998, N=48
> ratio*tsc >> N = 0.999... -> 0
> |
> kvm_get_time_scale
>
> In function __scale_tsc, it uses fixed point number to calculate
> tsc, therefore, a certain degree of precision is lost, the actual tsc
> value of 0.999... would be 0. In function kvm_get_time_scale
> tps32=tps64=base_hz=0, would lead second while_loop infinite. when
> CONFIG_PREEMPT is n, it causes a soft lockup issue.
>
> Fixes: 35181e86df97 ("KVM: x86: Add a common TSC scaling function")
> Signed-off-by: Yuntao Liu <liuyuntao12@xxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1fa5d89f8d27..3e9d6f368eed 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2605,10 +2605,14 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
> * point number (mult + frac * 2^(-N)).
> *
> * N equals to kvm_caps.tsc_scaling_ratio_frac_bits.
> + *
> + * return 1 if _tsc is 0.
> */
> static inline u64 __scale_tsc(u64 ratio, u64 tsc)
> {
> - return mul_u64_u64_shr(tsc, ratio, kvm_caps.tsc_scaling_ratio_frac_bits);
> + u64 _tsc = mul_u64_u64_shr(tsc, ratio, kvm_caps.tsc_scaling_ratio_frac_bits);
> +
> + return !_tsc ? 1 : _tsc;
This can be
return _tsc ? : 1;
However, I'm 99% certain this only affects kvm_guest_time_update(), because it's
the only code that scales a TSC *frequency*, versus scaling a TSC value. Hmm,
kvm_x86_vendor_init() also scales a frequency, but the multiplier and shift are
KVM controlled, so that calculation can never be '0.
So I think just this for a fix? Because in all other cases, a result of '0' is
totally fine, and arguably even more correct, e.g. when used in adjust_tsc_offset_host().
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b58a74c1722d..de51dbd85a58 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3258,9 +3258,11 @@ int kvm_guest_time_update(struct kvm_vcpu *v)
/* With all the info we got, fill in the values */
- if (kvm_caps.has_tsc_control)
+ if (kvm_caps.has_tsc_control) {
tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz,
v->arch.l1_tsc_scaling_ratio);
+ tgt_tsc_khz = tgt_tsc_khz ? : 1;
+ }
if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
kvm_get_time_scale(NSEC_PER_SEC, tgt_tsc_khz * 1000LL,
