Re: [RFC PATCH v3 13/21] KVM: x86: Improve synchronization in kvm_synchronize_tsc()

[Sean Christopherson <seanjc@google.com>]

On Wed, May 22, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> 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@amazon.co.uk>
> ---
>  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();
> +	}
> +