linux-kernel - Re: [PATCH v4 1/4] KVM: x86/mmu: Retry fault before acquiring mmu

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Date: Mon, 19 Feb 2024 08:18:12 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Xu Yilun <yilun.xu@...ux.intel.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Yan Zhao <yan.y.zhao@...el.com>, Friedrich Weber <f.weber@...xmox.com>, 
	Kai Huang <kai.huang@...el.com>, Yuan Yao <yuan.yao@...ux.intel.com>, 
	Yu Zhang <yu.c.zhang@...ux.intel.com>, Chao Peng <chao.p.peng@...ux.intel.com>, 
	Fuad Tabba <tabba@...gle.com>, Michael Roth <michael.roth@....com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH v4 1/4] KVM: x86/mmu: Retry fault before acquiring
 mmu_lock if mapping is changing

On Sat, Feb 17, 2024, Xu Yilun wrote:
> >  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 &&

Ugh, and bad testing on my end.