linux-kernel - Re: [PATCH v2 9/9] KVM: x86: never write to memory from kvm_vcpu_check

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Date:   Tue, 16 Aug 2022 23:45:08 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        mlevitsk@...hat.com, vkuznets@...hat.com
Subject: Re: [PATCH v2 9/9] KVM: x86: never write to memory from
 kvm_vcpu_check_block

On Thu, Aug 11, 2022, Paolo Bonzini wrote:
> kvm_vcpu_check_block() is called while not in TASK_RUNNING, and therefore
> it cannot sleep.  Writing to guest memory is therefore forbidden, but it
> can happen on AMD processors if kvm_check_nested_events() causes a vmexit.
> 
> Fortunately, all events that are caught by kvm_check_nested_events() are
> also recognized by kvm_vcpu_has_events() through vendor callbacks such as
> kvm_x86_interrupt_allowed() or kvm_x86_ops.nested_ops->has_events(), so
> remove the call and postpone the actual processing to vcpu_block().
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5e9358ea112b..9226fd536783 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10639,6 +10639,17 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
>  			return 1;
>  	}
>  
> +	if (is_guest_mode(vcpu)) {
> +		/*
> +		 * Evaluate nested events before exiting the halted state.
> +		 * This allows the halt state to be recorded properly in
> +		 * the VMCS12's activity state field (AMD does not have
> +		 * a similar field and a vmexit always causes a spurious
> +		 * wakeup from HLT).
> +		 */
> +		kvm_check_nested_events(vcpu);

Formatting nit, I'd prefer the block comment go above the if-statement, that way
we avoiding debating whether or not the technically-unnecessary braces align with
kernel/KVM style, and it doesn't have to wrap as aggressively.

And s/vmexit/VM-Exit while I'm nitpicking.

	/*
	 * Evaluate nested events before exiting the halted state.  This allows
	 * the halt state to be recorded properly in the VMCS12's activity
	 * state field (AMD does not have a similar field and a VM-Exit always
	 * causes a spurious wakeup from HLT).
	 */
	if (is_guest_mode(vcpu))
		kvm_check_nested_events(vcpu);

Side topic, the AMD behavior is a bug report waiting to happen.  I know of at least
one customer failure that was root caused to a KVM bug where KVM caused a spurious
wakeup.  To be fair, the guest workload was being stupid (execute HLT on vCPU and
then effectively unmap its code by doing kexec), but it's still an unpleasant gap :-(