linux-kernel - Re: [PATCH v2] KVM: x86/mmu: Retry fault before acquiring mmu

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Date: Fri, 19 Jan 2024 00:38:36 +0800
From: Xu Yilun <yilun.xu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Yan Zhao <yan.y.zhao@...el.com>,
	Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH v2] KVM: x86/mmu: Retry fault before acquiring mmu_lock
 if mapping is changing

On Tue, Jan 09, 2024 at 05:20:45PM -0800, Sean Christopherson wrote:
> Retry page faults without acquiring mmu_lock if the resolved gfn is covered
> by an active invalidation.  Contending for mmu_lock is especially
> problematic on preemptible kernels as the mmu_notifier invalidation task
> will yield mmu_lock (see rwlock_needbreak()), delay the in-progress

Is it possible fault-in task avoids contending mmu_lock by using _trylock()?
Like:

	while (!read_trylock(&vcpu->kvm->mmu_lock))
		cpu_relax();

	if (is_page_fault_stale(vcpu, fault))
		goto out_unlock;
  
	r = kvm_tdp_mmu_map(vcpu, fault);

out_unlock:
	read_unlock(&vcpu->kvm->mmu_lock)

> invalidation, and ultimately increase the latency of resolving the page
> fault.  And in the worst case scenario, yielding will be accompanied by a
> remote TLB flush, e.g. if the invalidation covers a large range of memory
> and vCPUs are accessing addresses that were already zapped.

This case covers all usage of mmu_invalidate_retry_gfn(), is it? Should
we also consider vmx_set_apic_access_page_addr()?

Thanks,
Yilun