On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> These pointlessly duplicate the last_tsc_{nsec,offset,write} values.
>
> The only place they were used was where the TSC is stable and a new vCPU
> is being synchronized to the previous setting, in which case the 'last_'
> value is definitely identical.
>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 3 ---
> arch/x86/kvm/x86.c | 19 ++++++++-----------
> 2 files changed, 8 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index b01c1d000fff..7d06f389a607 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1354,9 +1354,6 @@ struct kvm_arch {
> u32 last_tsc_khz;
> u64 last_tsc_offset;
> u64 last_tsc_scaling_ratio;
> - u64 cur_tsc_nsec;
> - u64 cur_tsc_write;
> - u64 cur_tsc_offset;
> u64 cur_tsc_generation;
> int nr_vcpus_matched_tsc;
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6ec43f39bdb0..ab5d55071253 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2713,11 +2713,9 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
> lockdep_assert_held(&kvm->arch.tsc_write_lock);
>
> /*
> - * We also track th most recent recorded KHZ, write and time to
> - * allow the matching interval to be extended at each write.
> + * Track the last recorded kHz (and associated scaling ratio for
> + * calculating the guest TSC), and offset.
> */
> - kvm->arch.last_tsc_nsec = ns;
> - kvm->arch.last_tsc_write = tsc;
> kvm->arch.last_tsc_khz = vcpu->arch.virtual_tsc_khz;
> kvm->arch.last_tsc_scaling_ratio = vcpu->arch.l1_tsc_scaling_ratio;
> kvm->arch.last_tsc_offset = offset;
> @@ -2736,10 +2734,9 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc,
> *
> * These values are tracked in kvm->arch.cur_xxx variables.
This comment is now stale, as most of the fields are now .last_xxx, not cur_xxx.
However...
> */
> + kvm->arch.last_tsc_nsec = ns;
There is a functional change here, and it's either incorrect or misleading (I
think the latter). If the TSC is unstable, "ns" in kvm_synchronize_tsc() will
come from get_kvmclock_base_ns(), and only the TSC frequency is checked for a
match when synchronizing.
That results in .last_tsc_nsec not being updated, and so subsequent syncs will
compute a larger elapsed time (relative to the current generation's timestamp,
not the "last" timestamp).
Functionally, I think that's ok? So long as all vCPUs sync against the same
baseline, it should work? I think.
But if that's the case, then I would prefer to delete last_tsc_{nsec,write,offset},
not the cur_xxx versions. For nsec and write it shows that they are valid/used
only in the context of the current generation.
And for the offset, updating it _outside_ of the loop makes it more obvious that
the offset can change (by design) within a generation if the TSC is unstable.
Ooh, and if I'm reading the code correctly, last_tsc_khz can be renamed to
cur_tsc_khz and moved in the !matched statement too, as it's guaranteed to be
vcpu->arch.virtual_tsc_khz if matched==true.
Ah, right, and last_tsc_scaling_ratio is just an deriviation of virtual_tsc_khz,
so it too can be cur_xxx and put under !matched.
Am I missing something? That seems too easy...
