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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Suleiman Souhlal <suleiman@google.com>
Cc:     ssouhlal@FreeBSD.org, joelaf@google.com, senozhatsky@chromium.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Suleiman Souhlal <suleiman@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [RFC PATCH 2/2] kvm,x86: Report preempt_count to host.
In-Reply-To: <20210728073700.120449-3-suleiman@google.com>
References: <20210728073700.120449-1-suleiman@google.com>
 <20210728073700.120449-3-suleiman@google.com>
Date:   Wed, 28 Jul 2021 10:18:33 +0200
Message-ID: <87tuke6ecm.fsf@vitty.brq.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Suleiman Souhlal <suleiman@google.com> writes:

> When KVM_PREEMPT_COUNT_REPORTING is enabled, the host can use
> preempt_count to determine if the guest is in a critical section,
> if it also has CONFIG_KVM_HETEROGENEOUS_RT enabled, in order to
> use heterogeneous RT VCPU configurations.
>
> Signed-off-by: Suleiman Souhlal <suleiman@google.com>
> ---
>  arch/x86/Kconfig      | 11 +++++++++++
>  arch/x86/kernel/kvm.c | 10 ++++++++++
>  2 files changed, 21 insertions(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 49270655e827..d8b62789df57 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -846,6 +846,17 @@ config PARAVIRT_TIME_ACCOUNTING
>  config PARAVIRT_CLOCK
>  	bool
>  
> +config KVM_PREEMPT_COUNT_REPORTING
> +	bool "KVM preempt_count reporting to the host"
> +	depends on KVM_GUEST && PREEMPT_COUNT
> +	default n
> +	help
> +	  Select this option to enable KVM preempt_count reporting to the host,
> +	  which can be useful in cases where some VCPUs are RT and the rest
> +	  aren't.
> +
> +	  If in doubt, say N here.
> +
>  config JAILHOUSE_GUEST
>  	bool "Jailhouse non-root cell support"
>  	depends on X86_64 && PCI
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index a26643dc6bd6..7ec53ea3f979 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -363,6 +363,16 @@ static void kvm_guest_cpu_init(void)
>  
>  	if (has_steal_clock)
>  		kvm_register_steal_time();
> +
> +#ifdef CONFIG_KVM_PREEMPT_COUNT_REPORTING
> +	if (kvm_para_has_feature(KVM_FEATURE_PREEMPT_COUNT)) {
> +		unsigned long pa;
> +
> +		pa = slow_virt_to_phys(this_cpu_ptr(&__preempt_count)) |
> +		    KVM_MSR_ENABLED;
> +		wrmsrl(MSR_KVM_PREEMPT_COUNT, pa);
> +	}
> +#endif
>  }

Please also disable the feature in kvm_guest_cpu_offline() as e.g. upon
kexec the memory address looses its meaning.

>  
>  static void kvm_pv_disable_apf(void)

-- 
Vitaly