linux-kernel - Re: [PATCH][v2] x86/kvm: Prefer native qspinlock for dedicated vCPUs irrespective of PV

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Message-ID: <aJ-KQ5811s2E5Dj9@google.com>
Date: Fri, 15 Aug 2025 12:28:03 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: lirongqing <lirongqing@...du.com>
Cc: pbonzini@...hat.com, vkuznets@...hat.com, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][v2] x86/kvm: Prefer native qspinlock for dedicated vCPUs
 irrespective of PV_UNHALT

On Tue, Jul 22, 2025, lirongqing wrote:
> From: Li RongQing <lirongqing@...du.com>
> 
> The commit b2798ba0b876 ("KVM: X86: Choose qspinlock when dedicated
> physical CPUs are available") states that when PV_DEDICATED=1
> (vCPU has dedicated pCPU), qspinlock should be preferred regardless of
> PV_UNHALT.  However, the current implementation doesn't reflect this: when
> PV_UNHALT=0, we still use virt_spin_lock() even with dedicated pCPUs.
> 
> This is suboptimal because:
> 1. Native qspinlocks should outperform virt_spin_lock() for dedicated
>    vCPUs irrespective of HALT exiting
> 2. virt_spin_lock() should only be preferred when vCPUs may be preempted
>    (non-dedicated case)
> 
> So reorder the PV spinlock checks to:
> 1. First handle dedicated pCPU case (disable virt_spin_lock_key)
> 2. Second check single CPU, and nopvspin configuration
> 3. Only then check PV_UNHALT support
> 
> This ensures we always use native qspinlock for dedicated vCPUs, delivering
> pretty performance gains at high contention levels.
> 
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@...gle.com>