Re: [PATCH] KVM: x86/mmu: add lockdep check before lookup_address_in_mm()

From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>,
	Mingwei Zhang <mizhang@google.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm <kvm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	Ben Gardon <bgardon@google.com>,
	David Matlack <dmatlack@google.com>
Subject: Re: [PATCH] KVM: x86/mmu: add lockdep check before lookup_address_in_mm()
Date: Tue, 26 Apr 2022 19:49:20 +0200	[thread overview]
Message-ID: <7597fe2c-ce04-0e21-bd6c-4051d7d5101d@redhat.com> (raw)
In-Reply-To: <YkH7KZbamhKpCidK@google.com>

On 3/28/22 20:15, Sean Christopherson wrote:
>> lookup_address_in_mm() walks the host page table as if it is a
>> sequence of_static_  memory chunks. This is clearly dangerous.
> Yeah, it's broken.  The proper fix is do something like what perf uses, or maybe
> just genericize and reuse the code from commit 8af26be06272
> ("perf/core: Fix arch_perf_get_page_size()).
> 

Indeed, KVM could use perf_get_pgtable_size().  The conversion from the 
result of *_leaf_size() to level is basically (ctz(size) - 12) / 9.

Alternatively, there are the three difference between 
perf_get_page_size() and lookup_address_in_pgd():

* the *_offset_lockless() macros, which are unnecessary on x86

* READ_ONCE, which is important but in practice unlikely to make a 
difference

* local_irq_{save,restore} around the walk

The last is the important one and it should be added to 
lookup_address_in_pgd().

Paolo