I have encountered this build error with this patch.
Perhaps it is because all usage of "flags" are removed.
$ make -j32 > /dev/null
arch/x86/kvm/x86.c: In function ‘kvm_guest_time_update’:
arch/x86/kvm/x86.c:3359:23: error: unused variable ‘flags’
[-Werror=unused-variable]
3359 | unsigned long flags;
| ^~~~~
cc1: all warnings being treated as errors
make[4]: *** [scripts/Makefile.build:289: arch/x86/kvm/x86.o] Error 1
make[3]: *** [scripts/Makefile.build:548: arch/x86/kvm] Error 2
make[2]: *** [scripts/Makefile.build:548: arch/x86] Error 2
make[1]: *** [/home/opc/ext4/mainline-linux/Makefile:2143: .] Error 2
make: *** [Makefile:248: __sub-make] Error 2
Thank you very much!
Dongli Zhang
On 2026-05-09 3:46 PM, David Woodhouse wrote:
> From: David Woodhouse <[email protected]>
>
> Restructure kvm_guest_time_update() so that kernel_ns/host_tsc are
> always "now" when doing TSC catchup, then swap in the master clock
> reference values afterward for the hv_clock.
>
> This makes the TSC upscaling code considerably simpler: the catchup
> adjustment is computed as the delta between what the guest TSC *should*
> be at "now" and what it actually is, rather than mixing "now" and
> "master clock reference" timestamps.
>
> The seqcount loop now also contains the kvm_get_time_and_clockread()
> call (matching get_kvmclock's pattern), with the same WARN for
> unexpected failure.
>
> Based on a suggestion by Sean Christopherson.
>
> Signed-off-by: David Woodhouse <[email protected]>
> ---
> arch/x86/kvm/x86.c | 67 ++++++++++++++++++++++++++++++++--------------
> 1 file changed, 47 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e281c49561fa..8e4993ef4f6b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3363,39 +3363,51 @@ int kvm_guest_time_update(struct kvm_vcpu *v)
> struct kvm_arch *ka = &v->kvm->arch;
> s64 kernel_ns;
> u64 tsc_timestamp, host_tsc;
> + u64 master_host_tsc = 0;
> + s64 master_kernel_ns = 0;
> bool use_master_clock;
>
> - kernel_ns = 0;
> - host_tsc = 0;
> -
> /*
> * If the host uses TSC clock, then passthrough TSC as stable
> * to the guest.
> */
> do {
> seq = read_seqcount_begin(&ka->pvclock_sc);
> +
> use_master_clock = ka->use_master_clock;
> - if (use_master_clock) {
> - host_tsc = ka->master_cycle_now;
> - kernel_ns = ka->master_kernel_ns;
> - }
> +
> + /*
> + * The TSC read and the call to get_cpu_tsc_khz() must happen
> + * on the same CPU.
> + */
> + get_cpu();
> +
> + tgt_tsc_hz = (u64)get_cpu_tsc_khz() * 1000;
> +
> + if (use_master_clock &&
> + !kvm_get_time_and_clockread(&kernel_ns, &host_tsc) &&
> + WARN_ON_ONCE(!read_seqcount_retry(&ka->pvclock_sc, seq)))
> + use_master_clock = false;
> +
> + put_cpu();
> +
> + if (!use_master_clock)
> + break;
> +
> + master_host_tsc = ka->master_cycle_now;
> + master_kernel_ns = ka->master_kernel_ns;
> } while (read_seqcount_retry(&ka->pvclock_sc, seq));
>
> - /* Keep irq disabled to prevent changes to the clock */
> - local_irq_save(flags);
> - tgt_tsc_hz = (u64)get_cpu_tsc_khz() * 1000;
> if (unlikely(tgt_tsc_hz == 0)) {
> - local_irq_restore(flags);
> kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
> return 1;
> }
> +
> if (!use_master_clock) {
> host_tsc = rdtsc();
> kernel_ns = get_kvmclock_base_ns();
> }
>
> - tsc_timestamp = kvm_read_l1_tsc(v, host_tsc);
> -
> /*
> * We may have to catch up the TSC to match elapsed wall clock
> * time for two reasons, even if kvmclock is used.
> @@ -3404,17 +3416,32 @@ int kvm_guest_time_update(struct kvm_vcpu *v)
> * entry to avoid unknown leaps of TSC even when running
> * again on the same CPU. This may cause apparent elapsed
> * time to disappear, and the guest to stand still or run
> - * very slowly.
> + * very slowly.
> */
> if (vcpu->tsc_catchup) {
> - u64 tsc = compute_guest_tsc(v, kernel_ns);
> - if (tsc > tsc_timestamp) {
> - adjust_tsc_offset_guest(v, tsc - tsc_timestamp);
> - tsc_timestamp = tsc;
> - }
> + s64 adjustment;
> +
> + /*
> + * Calculate the delta between what the guest TSC *should* be
> + * and what it actually is according to kvm_read_l1_tsc().
> + */
> + adjustment = compute_guest_tsc(v, kernel_ns) -
> + kvm_read_l1_tsc(v, host_tsc);
> + if (adjustment > 0)
> + adjust_tsc_offset_guest(v, adjustment);
> }
>
> - local_irq_restore(flags);
> + /*
> + * Now that TSC upscaling is out of the way, the remaining calculations
> + * are all relative to the reference time that's placed in hv_clock.
> + * If the master clock is NOT in use, the reference time is "now". If
> + * master clock is in use, the reference time comes from there.
> + */
> + if (use_master_clock) {
> + host_tsc = master_host_tsc;
> + kernel_ns = master_kernel_ns;
> + }
> + tsc_timestamp = kvm_read_l1_tsc(v, host_tsc);
>
> /* With all the info we got, fill in the values */
>
