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Subject: Re: [PATCH v2 2/7] kvm: add guest_state_{enter,exit}_irqoff()
From:   Nicolas Saenz Julienne <nsaenzju@redhat.com>
To:     Mark Rutland <mark.rutland@arm.com>, linux-kernel@vger.kernel.org
Cc:     aleksandar.qemu.devel@gmail.com, alexandru.elisei@arm.com,
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Date:   Fri, 21 Jan 2022 18:35:37 +0100
In-Reply-To: <20220119105854.3160683-3-mark.rutland@arm.com>
References: <20220119105854.3160683-1-mark.rutland@arm.com>
         <20220119105854.3160683-3-mark.rutland@arm.com>
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On Wed, 2022-01-19 at 10:58 +0000, Mark Rutland wrote:
> When transitioning to/from guest mode, it is necessary to inform
> lockdep, tracing, and RCU in a specific order, similar to the
> requirements for transitions to/from user mode. Additionally, it is
> necessary to perform vtime accounting for a window around running the
> guest, with RCU enabled, such that timer interrupts taken from the guest
> can be accounted as guest time.
> 
> Most architectures don't handle all the necessary pieces, and a have a
> number of common bugs, including unsafe usage of RCU during the window
> between guest_enter() and guest_exit().
> 
> On x86, this was dealt with across commits:
> 
>   87fa7f3e98a1310e ("x86/kvm: Move context tracking where it belongs")
>   0642391e2139a2c1 ("x86/kvm/vmx: Add hardirq tracing to guest enter/exit")
>   9fc975e9efd03e57 ("x86/kvm/svm: Add hardirq tracing on guest enter/exit")
>   3ebccdf373c21d86 ("x86/kvm/vmx: Move guest enter/exit into .noinstr.text")
>   135961e0a7d555fc ("x86/kvm/svm: Move guest enter/exit into .noinstr.text")
>   160457140187c5fb ("KVM: x86: Defer vtime accounting 'til after IRQ handling")
>   bc908e091b326467 ("KVM: x86: Consolidate guest enter/exit logic to common helpers")
> 
> ... but those fixes are specific to x86, and as the resulting logic
> (while correct) is split across generic helper functions and
> x86-specific helper functions, it is difficult to see that the
> entry/exit accounting is balanced.
> 
> This patch adds generic helpers which architectures can use to handle
> guest entry/exit consistently and correctly. The guest_{enter,exit}()
> helpers are split into guest_timing_{enter,exit}() to perform vtime
> accounting, and guest_context_{enter,exit}() to perform the necessary
> context tracking and RCU management. The existing guest_{enter,exit}()
> heleprs are left as wrappers of these.
> 
> Atop this, new guest_state_enter_irqoff() and guest_state_exit_irqoff()
> helpers are added to handle the ordering of lockdep, tracing, and RCU
> manageent. These are inteneded to mirror exit_to_user_mode() and
> enter_from_user_mode().
> 
> Subsequent patches will migrate architectures over to the new helpers,
> following a sequence:
> 
> 	guest_timing_enter_irqoff();
> 
> 	guest_state_enter_irqoff();
> 	< run the vcpu >
> 	guest_state_exit_irqoff();
> 
> 	< take any pending IRQs >
> 
> 	guest_timing_exit_irqoff();
> 
> This sequences handles all of the above correctly, and more clearly
> balances the entry and exit portions, making it easier to understand.
> 
> The existing helpers are marked as deprecated, and will be removed once
> all architectures have been converted.
> 
> There should be no functional change as a result of this patch.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Reviewed-by: Marc Zyngier <maz@kernel.org>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> ---

Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>

Thanks,

-- 
Nicolás Sáenz