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From: Sean Christopherson <seanjc@google.com>
To: Will Deacon <will@kernel.org>
Cc: Alexandru Elisei <alexandru.elisei@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	kvmarm@lists.cs.columbia.edu, linux-mm@kvack.org,
	Matthew Wilcox <willy@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Quentin Perret <qperret@google.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	kernel-team@android.com
Subject: Re: [PATCH 1/5] KVM: arm64: Walk userspace page tables to compute the THP mapping size
Date: Wed, 21 Jul 2021 15:56:09 +0000	[thread overview]
Message-ID: <YPhDmZya2Up7fYNN@google.com> (raw)
In-Reply-To: <20210721145828.GA11003@willie-the-truck>

On Wed, Jul 21, 2021, Will Deacon wrote:
> > For the page tables liveliness, KVM implements mmu_notifier_ops.release, which is
> > invoked at the beginning of exit_mmap(), before the page tables are freed.  In
> > its implementation, KVM takes mmu_lock and zaps all its shadow page tables, a.k.a.
> > the stage2 tables in KVM arm64.  The flow in question, get_user_mapping_size(),
> > also runs under mmu_lock, and so effectively blocks exit_mmap() and thus is
> > guaranteed to run with live userspace tables.
> 
> Unless I missed a case, exit_mmap() only runs when mm_struct::mm_users drops
> to zero, right?

Yep.

> The vCPU tasks should hold references to that afaict, so I don't think it
> should be possible for exit_mmap() to run while there are vCPUs running with
> the corresponding page-table.

Ah, right, I was thinking of non-KVM code that operated on the page tables without
holding a reference to mm_users.

> > Looking at the arm64 code, one thing I'm not clear on is whether arm64 correctly
> > handles the case where exit_mmap() wins the race.  The invalidate_range hooks will
> > still be called, so userspace page tables aren't a problem, but
> > kvm_arch_flush_shadow_all() -> kvm_free_stage2_pgd() nullifies mmu->pgt without
> > any additional notifications that I see.  x86 deals with this by ensuring its
> > top-level TDP entry (stage2 equivalent) is valid while the page fault handler is
> > running.
> 
> But the fact that x86 handles this race has me worried. What am I missing?

I don't think you're missing anything.  I forgot that KVM_RUN would require an
elevated mm_users.  x86 does handle the impossible race, but that's coincidental.
The extra protections in x86 are to deal with other cases where a vCPU's top-level
SPTE can be invalidated while the vCPU is running.

WARNING: multiple messages have this Message-ID
From: Sean Christopherson <seanjc@google.com>
To: Will Deacon <will@kernel.org>
Cc: kernel-team@android.com, kvm@vger.kernel.org,
	Marc Zyngier <maz@kernel.org>,
	Matthew Wilcox <willy@infradead.org>,
	linux-mm@kvack.org, Paolo Bonzini <pbonzini@redhat.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/5] KVM: arm64: Walk userspace page tables to compute the THP mapping size
Date: Wed, 21 Jul 2021 15:56:09 +0000	[thread overview]
Message-ID: <YPhDmZya2Up7fYNN@google.com> (raw)
In-Reply-To: <20210721145828.GA11003@willie-the-truck>

On Wed, Jul 21, 2021, Will Deacon wrote:
> > For the page tables liveliness, KVM implements mmu_notifier_ops.release, which is
> > invoked at the beginning of exit_mmap(), before the page tables are freed.  In
> > its implementation, KVM takes mmu_lock and zaps all its shadow page tables, a.k.a.
> > the stage2 tables in KVM arm64.  The flow in question, get_user_mapping_size(),
> > also runs under mmu_lock, and so effectively blocks exit_mmap() and thus is
> > guaranteed to run with live userspace tables.
> 
> Unless I missed a case, exit_mmap() only runs when mm_struct::mm_users drops
> to zero, right?

Yep.

> The vCPU tasks should hold references to that afaict, so I don't think it
> should be possible for exit_mmap() to run while there are vCPUs running with
> the corresponding page-table.

Ah, right, I was thinking of non-KVM code that operated on the page tables without
holding a reference to mm_users.

> > Looking at the arm64 code, one thing I'm not clear on is whether arm64 correctly
> > handles the case where exit_mmap() wins the race.  The invalidate_range hooks will
> > still be called, so userspace page tables aren't a problem, but
> > kvm_arch_flush_shadow_all() -> kvm_free_stage2_pgd() nullifies mmu->pgt without
> > any additional notifications that I see.  x86 deals with this by ensuring its
> > top-level TDP entry (stage2 equivalent) is valid while the page fault handler is
> > running.
> 
> But the fact that x86 handles this race has me worried. What am I missing?

I don't think you're missing anything.  I forgot that KVM_RUN would require an
elevated mm_users.  x86 does handle the impossible race, but that's coincidental.
The extra protections in x86 are to deal with other cases where a vCPU's top-level
SPTE can be invalidated while the vCPU is running.
_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

WARNING: multiple messages have this Message-ID
From: Sean Christopherson <seanjc@google.com>
To: Will Deacon <will@kernel.org>
Cc: Alexandru Elisei <alexandru.elisei@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	kvmarm@lists.cs.columbia.edu, linux-mm@kvack.org,
	Matthew Wilcox <willy@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Quentin Perret <qperret@google.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	kernel-team@android.com
Subject: Re: [PATCH 1/5] KVM: arm64: Walk userspace page tables to compute the THP mapping size
Date: Wed, 21 Jul 2021 15:56:09 +0000	[thread overview]
Message-ID: <YPhDmZya2Up7fYNN@google.com> (raw)
In-Reply-To: <20210721145828.GA11003@willie-the-truck>

On Wed, Jul 21, 2021, Will Deacon wrote:
> > For the page tables liveliness, KVM implements mmu_notifier_ops.release, which is
> > invoked at the beginning of exit_mmap(), before the page tables are freed.  In
> > its implementation, KVM takes mmu_lock and zaps all its shadow page tables, a.k.a.
> > the stage2 tables in KVM arm64.  The flow in question, get_user_mapping_size(),
> > also runs under mmu_lock, and so effectively blocks exit_mmap() and thus is
> > guaranteed to run with live userspace tables.
> 
> Unless I missed a case, exit_mmap() only runs when mm_struct::mm_users drops
> to zero, right?

Yep.

> The vCPU tasks should hold references to that afaict, so I don't think it
> should be possible for exit_mmap() to run while there are vCPUs running with
> the corresponding page-table.

Ah, right, I was thinking of non-KVM code that operated on the page tables without
holding a reference to mm_users.

> > Looking at the arm64 code, one thing I'm not clear on is whether arm64 correctly
> > handles the case where exit_mmap() wins the race.  The invalidate_range hooks will
> > still be called, so userspace page tables aren't a problem, but
> > kvm_arch_flush_shadow_all() -> kvm_free_stage2_pgd() nullifies mmu->pgt without
> > any additional notifications that I see.  x86 deals with this by ensuring its
> > top-level TDP entry (stage2 equivalent) is valid while the page fault handler is
> > running.
> 
> But the fact that x86 handles this race has me worried. What am I missing?

I don't think you're missing anything.  I forgot that KVM_RUN would require an
elevated mm_users.  x86 does handle the impossible race, but that's coincidental.
The extra protections in x86 are to deal with other cases where a vCPU's top-level
SPTE can be invalidated while the vCPU is running.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  reply	other threads:[~2021-07-21 15:56 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-17  9:55 [PATCH 0/5] KVM: Remove kvm_is_transparent_hugepage() and friends Marc Zyngier
2021-07-17  9:55 ` Marc Zyngier
2021-07-17  9:55 ` Marc Zyngier
2021-07-17  9:55 ` [PATCH 1/5] KVM: arm64: Walk userspace page tables to compute the THP mapping size Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-19  6:31   ` Paolo Bonzini
2021-07-19  6:31     ` Paolo Bonzini
2021-07-19  6:31     ` Paolo Bonzini
2021-07-19  9:31     ` Marc Zyngier
2021-07-19  9:31       ` Marc Zyngier
2021-07-19  9:31       ` Marc Zyngier
2021-07-20 17:23   ` Alexandru Elisei
2021-07-20 17:23     ` Alexandru Elisei
2021-07-20 17:23     ` Alexandru Elisei
2021-07-20 20:33     ` Sean Christopherson
2021-07-20 20:33       ` Sean Christopherson
2021-07-20 20:33       ` Sean Christopherson
2021-07-21 14:58       ` Will Deacon
2021-07-21 14:58         ` Will Deacon
2021-07-21 14:58         ` Will Deacon
2021-07-21 15:56         ` Sean Christopherson [this message]
2021-07-21 15:56           ` Sean Christopherson
2021-07-21 15:56           ` Sean Christopherson
2021-07-21 16:37       ` Alexandru Elisei
2021-07-21 16:37         ` Alexandru Elisei
2021-07-21 16:37         ` Alexandru Elisei
2021-07-23  8:48     ` Marc Zyngier
2021-07-23  8:48       ` Marc Zyngier
2021-07-23  8:48       ` Marc Zyngier
2021-07-17  9:55 ` [PATCH 2/5] KVM: arm64: Avoid mapping size adjustment on permission fault Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-23 15:55   ` Alexandru Elisei
2021-07-23 15:55     ` Alexandru Elisei
2021-07-23 15:55     ` Alexandru Elisei
2021-07-23 16:18     ` Marc Zyngier
2021-07-23 16:18       ` Marc Zyngier
2021-07-23 16:18       ` Marc Zyngier
2021-07-17  9:55 ` [PATCH 3/5] KVM: Remove kvm_is_transparent_hugepage() and PageTransCompoundMap() Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-19  6:31   ` Paolo Bonzini
2021-07-19  6:31     ` Paolo Bonzini
2021-07-19  6:31     ` Paolo Bonzini
2021-07-17  9:55 ` [PATCH 4/5] KVM: arm64: Use get_page() instead of kvm_get_pfn() Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-17  9:55 ` [PATCH 5/5] KVM: Get rid " Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-17  9:55   ` Marc Zyngier
2021-07-19  6:31   ` Paolo Bonzini
2021-07-19  6:31     ` Paolo Bonzini
2021-07-19  6:31     ` Paolo Bonzini

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