Re: [PATCH v7 4/6] KVM: x86: Take the pvclock sync lock behind the tsc_write_lock

From: Sean Christopherson <seanjc@google.com>
To: Oliver Upton <oupton@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	kvm@vger.kernel.org, Will Deacon <will@kernel.org>,
	Marc Zyngier <maz@kernel.org>, Peter Shier <pshier@google.com>,
	David Matlack <dmatlack@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org,
	Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH v7 4/6] KVM: x86: Take the pvclock sync lock behind the tsc_write_lock
Date: Thu, 2 Sep 2021 19:22:39 +0000	[thread overview]
Message-ID: <YTEkf3wn/PbVpYni@google.com> (raw)
In-Reply-To: <20210816001130.3059564-5-oupton@google.com>

On Mon, Aug 16, 2021, Oliver Upton wrote:
> A later change requires that the pvclock sync lock be taken while
> holding the tsc_write_lock. Change the locking in kvm_synchronize_tsc()
> to align with the requirement to isolate the locking change to its own
> commit.
> 
> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
>  Documentation/virt/kvm/locking.rst | 11 +++++++++++
>  arch/x86/kvm/x86.c                 |  2 +-
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
> index 8138201efb09..0bf346adac2a 100644
> --- a/Documentation/virt/kvm/locking.rst
> +++ b/Documentation/virt/kvm/locking.rst
> @@ -36,6 +36,9 @@ On x86:
>    holding kvm->arch.mmu_lock (typically with ``read_lock``, otherwise
>    there's no need to take kvm->arch.tdp_mmu_pages_lock at all).
>  
> +- kvm->arch.tsc_write_lock is taken outside
> +  kvm->arch.pvclock_gtod_sync_lock
> +
>  Everything else is a leaf: no other lock is taken inside the critical
>  sections.
>  
> @@ -222,6 +225,14 @@ time it will be set using the Dirty tracking mechanism described above.
>  :Comment:	'raw' because hardware enabling/disabling must be atomic /wrt
>  		migration.
>  
> +:Name:		kvm_arch::pvclock_gtod_sync_lock
> +:Type:		raw_spinlock_t
> +:Arch:		x86
> +:Protects:	kvm_arch::{cur_tsc_generation,cur_tsc_nsec,cur_tsc_write,
> +			cur_tsc_offset,nr_vcpus_matched_tsc}
> +:Comment:	'raw' because updating the kvm master clock must not be
> +		preempted.
> +
>  :Name:		kvm_arch::tsc_write_lock
>  :Type:		raw_spinlock
>  :Arch:		x86
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b1e9a4885be6..f1434cd388b9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2533,7 +2533,6 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
>  	vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
>  
>  	kvm_vcpu_write_tsc_offset(vcpu, offset);
> -	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
>  
>  	spin_lock_irqsave(&kvm->arch.pvclock_gtod_sync_lock, flags);
>  	if (!matched) {
> @@ -2544,6 +2543,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
>  
>  	kvm_track_tsc_matching(vcpu);
>  	spin_unlock_irqrestore(&kvm->arch.pvclock_gtod_sync_lock, flags);

Drop the irqsave/irqrestore in this patch instead of doing so while refactoring
the code in the next patch.

> +	raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
>  }
>  
>  static inline void adjust_tsc_offset_guest(struct kvm_vcpu *vcpu,
> -- 
> 2.33.0.rc1.237.g0d66db33f3-goog
> 
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