linux-kernel - Re: [PATCH] x86/kvm: Don't use pv tlb/ipi/sched

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Message-ID: <60c796c5-9a24-8e2a-b417-f0d7259029b2@redhat.com>
Date:   Fri, 18 Feb 2022 09:36:29 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] x86/kvm: Don't use pv tlb/ipi/sched_yield if on 1 vCPU

On 2/18/22 09:10, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Inspired by commit 3553ae5690a (x86/kvm: Don't use pvqspinlock code if
> only 1 vCPU), on a VM with only 1 vCPU, there is no need to enable
> pv tlb/ipi/sched_yield and we can save the memory for __pv_cpu_mask.
> 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>   arch/x86/kernel/kvm.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index a438217cbfac..f734e3b0cfec 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -462,19 +462,22 @@ static bool pv_tlb_flush_supported(void)
>   {
>   	return (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH) &&
>   		!kvm_para_has_hint(KVM_HINTS_REALTIME) &&
> -		kvm_para_has_feature(KVM_FEATURE_STEAL_TIME));
> +		kvm_para_has_feature(KVM_FEATURE_STEAL_TIME) &&
> +		(num_possible_cpus() != 1));
>   }
>   
>   static bool pv_ipi_supported(void)
>   {
> -	return kvm_para_has_feature(KVM_FEATURE_PV_SEND_IPI);
> +	return (kvm_para_has_feature(KVM_FEATURE_PV_SEND_IPI) &&
> +	       (num_possible_cpus() != 1));
>   }
>   
>   static bool pv_sched_yield_supported(void)
>   {
>   	return (kvm_para_has_feature(KVM_FEATURE_PV_SCHED_YIELD) &&
>   		!kvm_para_has_hint(KVM_HINTS_REALTIME) &&
> -	    kvm_para_has_feature(KVM_FEATURE_STEAL_TIME));
> +	    kvm_para_has_feature(KVM_FEATURE_STEAL_TIME) &&
> +	    (num_possible_cpus() != 1));
>   }
>   
>   #define KVM_IPI_CLUSTER_SIZE	(2 * BITS_PER_LONG)

Queued, thanks.

Paolo