On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> When synchronizing to an existing TSC (either by explicitly writing zero,
> or the legacy hack where the TSC is written within one second's worth of
> the previously written TSC), the last_tsc_write and last_tsc_nsec values
> were being misrecorded by __kvm_synchronize_tsc(). The *unsynchronized*
> value of the TSC (perhaps even zero) was bring recorded, along with the
> current time at which kvm_synchronize_tsc() was called. This could cause
> *subsequent* writes to fail to synchronize correctly.
>
> Fix that by resetting {data, ns} to the previous values before passing
> them to __kvm_synchronize_tsc() when synchronization is detected. Except
> in the case where the TSC is unstable and *has* to be synthesised from
> the host clock, in which case attempt to create a nsec/tsc pair which is
> on the correct line.
>
> Furthermore, there were *three* different TSC reads used for calculating
> the "current" time, all slightly different from each other. Fix that by
> using kvm_get_time_and_clockread() where possible and using the same
> host_tsc value in all cases.
Please split this into two patches, one to switch to a single RDTSC, and another
do fix the other stuff.
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 32 ++++++++++++++++++++++++++++----
> 1 file changed, 28 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ea59694d712a..6ec43f39bdb0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -201,6 +201,10 @@ module_param(eager_page_split, bool, 0644);
> static bool __read_mostly mitigate_smt_rsb;
> module_param(mitigate_smt_rsb, bool, 0444);
>
> +#ifdef CONFIG_X86_64
> +static bool kvm_get_time_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp);
> +#endif
> +
> /*
> * Restoring the host value for MSRs that are only consumed when running in
> * usermode, e.g. SYSCALL MSRs and TSC_AUX, can be deferred until the CPU
> @@ -2753,14 +2757,22 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 *user_value)
> {
> u64 data = user_value ? *user_value : 0;
> struct kvm *kvm = vcpu->kvm;
> - u64 offset, ns, elapsed;
> + u64 offset, host_tsc, ns, elapsed;
> unsigned long flags;
> bool matched = false;
> bool synchronizing = false;
>
> +#ifdef CONFIG_X86_64
> + if (!kvm_get_time_and_clockread(&ns, &host_tsc))
> +#endif
I'm pretty sure we can unconditionally declare kvm_get_time_and_clockread() above,
and then do
if (!IS_ENABLED(CONFIG_X86_64) ||
!kvm_get_time_and_clockread(&ns, &host_tsc))
and let dead code elimintation do its thing to avoid a linker error.
> + {
> + ns = get_kvmclock_base_ns();
> + host_tsc = rdtsc();
> + }
> +
