linux-kernel - Re: [PATCH AUTOSEL 5.15 06/23] kvm: x86: Disable KVM_HC_CLOCK

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Date:   Tue, 1 Mar 2022 21:17:53 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Anton Romanov <romanton@...gle.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.15 06/23] kvm: x86: Disable KVM_HC_CLOCK_PAIRING
 if tsc is in always catchup mode

On 3/1/22 21:16, Sasha Levin wrote:
> From: Anton Romanov <romanton@...gle.com>
> 
> [ Upstream commit 3a55f729240a686aa8af00af436306c0cd532522 ]
> 
> If vcpu has tsc_always_catchup set each request updates pvclock data.
> KVM_HC_CLOCK_PAIRING consumers such as ptp_kvm_x86 rely on tsc read on
> host's side and do hypercall inside pvclock_read_retry loop leading to
> infinite loop in such situation.
> 
> v3:
>      Removed warn
>      Changed return code to KVM_EFAULT
> v2:
>      Added warn
> 
> Signed-off-by: Anton Romanov <romanton@...gle.com>
> Message-Id: <20220216182653.506850-1-romanton@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   arch/x86/kvm/x86.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 33cb065181248..c804122bb6e3a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8657,6 +8657,13 @@ static int kvm_pv_clock_pairing(struct kvm_vcpu *vcpu, gpa_t paddr,
>   	if (clock_type != KVM_CLOCK_PAIRING_WALLCLOCK)
>   		return -KVM_EOPNOTSUPP;
>   
> +	/*
> +	 * When tsc is in permanent catchup mode guests won't be able to use
> +	 * pvclock_read_retry loop to get consistent view of pvclock
> +	 */
> +	if (vcpu->arch.tsc_always_catchup)
> +		return -KVM_EOPNOTSUPP;
> +
>   	if (!kvm_get_walltime_and_clockread(&ts, &cycle))
>   		return -KVM_EOPNOTSUPP;
>   

Acked-by: Paolo Bonzini <pbonzini@...hat.com>