linux-kernel - Re: [RFC PATCH v3 13/21] KVM: x86: Improve synchronization in kvm_synchronize

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Message-ID: <Zr4mBNTSquDqOtfw@google.com>
Date: Thu, 15 Aug 2024 09:00:04 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Woodhouse <dwmw2@...radead.org>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
	Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Paul Durrant <paul@....org>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Daniel Bristot de Oliveira <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	jalliste@...zon.co.uk, sveith@...zon.de, zide.chen@...el.com, 
	Dongli Zhang <dongli.zhang@...cle.com>, Chenyi Qiang <chenyi.qiang@...el.com>
Subject: Re: [RFC PATCH v3 13/21] KVM: x86: Improve synchronization in kvm_synchronize_tsc()

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