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Date: Sat, 17 Feb 2024 22:45:54 +0800
From: Xu Yilun <yilun.xu@linux.intel.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, Yan Zhao <yan.y.zhao@intel.com>,
	Friedrich Weber <f.weber@proxmox.com>,
	Kai Huang <kai.huang@intel.com>,
	Yuan Yao <yuan.yao@linux.intel.com>,
	Yu Zhang <yu.c.zhang@linux.intel.com>,
	Chao Peng <chao.p.peng@linux.intel.com>,
	Fuad Tabba <tabba@google.com>, Michael Roth <michael.roth@amd.com>,
	Isaku Yamahata <isaku.yamahata@intel.com>,
	David Matlack <dmatlack@google.com>
Subject: Re: [PATCH v4 1/4] KVM: x86/mmu: Retry fault before acquiring
 mmu_lock if mapping is changing
Message-ID: <ZdDGooxx/a+sAzmq@yilunxu-OptiPlex-7050>
References: <20240209222858.396696-1-seanjc@google.com>
 <20240209222858.396696-2-seanjc@google.com>
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In-Reply-To: <20240209222858.396696-2-seanjc@google.com>

>  static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>  			   unsigned int access)
>  {
> +	struct kvm_memory_slot *slot = fault->slot;
>  	int ret;
>  
>  	fault->mmu_seq = vcpu->kvm->mmu_invalidate_seq;
>  	smp_rmb();
>  
> +	/*
> +	 * Check for a relevant mmu_notifier invalidation event before getting
> +	 * the pfn from the primary MMU, and before acquiring mmu_lock.
> +	 *
> +	 * For mmu_lock, if there is an in-progress invalidation and the kernel
> +	 * allows preemption, the invalidation task may drop mmu_lock and yield
> +	 * in response to mmu_lock being contended, which is *very* counter-
> +	 * productive as this vCPU can't actually make forward progress until
> +	 * the invalidation completes.
> +	 *
> +	 * Retrying now can also avoid unnessary lock contention in the primary
> +	 * MMU, as the primary MMU doesn't necessarily hold a single lock for
> +	 * the duration of the invalidation, i.e. faulting in a conflicting pfn
> +	 * can cause the invalidation to take longer by holding locks that are
> +	 * needed to complete the invalidation.
> +	 *
> +	 * Do the pre-check even for non-preemtible kernels, i.e. even if KVM
> +	 * will never yield mmu_lock in response to contention, as this vCPU is
> +	 * *guaranteed* to need to retry, i.e. waiting until mmu_lock is held
> +	 * to detect retry guarantees the worst case latency for the vCPU.
> +	 */
> +	if (!slot &&

typo?   if (slot &&

Thanks,
Yilun

> +	    mmu_invalidate_retry_gfn_unsafe(vcpu->kvm, fault->mmu_seq, fault->gfn))
> +		return RET_PF_RETRY;
> +
>  	ret = __kvm_faultin_pfn(vcpu, fault);
>  	if (ret != RET_PF_CONTINUE)
>  		return ret;